[FIX] Handle decimal and thousand separator in 'round_trip' converer
- [x] closes #35365
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

In case of non c-locale decimal and tsep, copy and fixup the source string before passing it to PyOS_string_to_double
Hello @ales-erjavec! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-08-10 10:12:58 UTC @gfyoung or @WillAyd if comments. @ales-erjavec can you also run the current csv asv's to see if perf is changed; we *might* need additional coverage for thousands sep as well. > ... can you also run the current csv asv's to see if perf is changed

`asv continuous -f 1.1 -E virtualenv --python 3.7  origin/master round-trip-decimal  -b ^io.csv`


<details>

```
· No executable found for python 3.6
· Creating environments
· Discovering benchmarks
·· Uninstalling from virtualenv-py3.7-Cython0.29.16-jinja2-matplotlib-numba-numexpr-numpy-odfpy-openpyxl-pytest-scipy-sqlalchemy-tables-xlrd-xlsxwriter-xlwt.
·· Installing 8703ad6e <round-trip-decimal> into virtualenv-py3.7-Cython0.29.16-jinja2-matplotlib-numba-numexpr-numpy-odfpy-openpyxl-pytest-scipy-sqlalchemy-tables-xlrd-xlsxwriter-xlwt..
· Running 48 total benchmarks (2 commits * 1 environments * 24 benchmarks)
[  0.00%] · For pandas commit aefae55e <master> (round 1/2):
[  0.00%] ·· Building for virtualenv-py3.7-Cython0.29.16-jinja2-matplotlib-numba-numexpr-numpy-odfpy-openpyxl-pytest-scipy-sqlalchemy-tables-xlrd-xlsxwriter-xlwt...
[  0.00%] ·· Benchmarking virtualenv-py3.7-Cython0.29.16-jinja2-matplotlib-numba-numexpr-numpy-odfpy-openpyxl-pytest-scipy-sqlalchemy-tables-xlrd-xlsxwriter-xlwt
[  1.04%] ··· Running (io.csv.ParseDateComparison.time_read_csv_dayfirst--)..............
[ 16.67%] ··· Running (io.csv.ReadCSVParseSpecialDate.time_read_special_date--).........
[ 25.00%] · For pandas commit 8703ad6e <round-trip-decimal> (round 1/2):
[ 25.00%] ·· Building for virtualenv-py3.7-Cython0.29.16-jinja2-matplotlib-numba-numexpr-numpy-odfpy-openpyxl-pytest-scipy-sqlalchemy-tables-xlrd-xlsxwriter-xlwt...
[ 25.00%] ·· Benchmarking virtualenv-py3.7-Cython0.29.16-jinja2-matplotlib-numba-numexpr-numpy-odfpy-openpyxl-pytest-scipy-sqlalchemy-tables-xlrd-xlsxwriter-xlwt
[ 26.04%] ··· Running (io.csv.ParseDateComparison.time_read_csv_dayfirst--)..............
[ 41.67%] ··· Running (io.csv.ReadCSVParseSpecialDate.time_read_special_date--).........
[ 50.00%] · For pandas commit 8703ad6e <round-trip-decimal> (round 2/2):
[ 50.00%] ·· Benchmarking virtualenv-py3.7-Cython0.29.16-jinja2-matplotlib-numba-numexpr-numpy-odfpy-openpyxl-pytest-scipy-sqlalchemy-tables-xlrd-xlsxwriter-xlwt
[ 51.04%] ··· io.csv.ParseDateComparison.time_read_csv_dayfirst                                                                                           ok
[ 51.04%] ··· ============= ============
               cache_dates              
              ------------- ------------
                  False      7.97±0.1ms 
                   True      3.94±0.1ms 
              ============= ============

[ 52.08%] ··· io.csv.ParseDateComparison.time_to_datetime_dayfirst                                                                                        ok
[ 52.08%] ··· ============= ============
               cache_dates              
              ------------- ------------
                  False      8.42±0.2ms 
                   True      3.83±0.2ms 
              ============= ============

[ 53.12%] ··· io.csv.ParseDateComparison.time_to_datetime_format_DD_MM_YYYY                                                                               ok
[ 53.12%] ··· ============= =============
               cache_dates               
              ------------- -------------
                  False        25.8±1ms  
                   True      3.76±0.05ms 
              ============= =============

[ 54.17%] ··· io.csv.ReadCSVCachedParseDates.time_read_csv_cached                                                                                         ok
[ 54.17%] ··· ========== =============
               do_cache               
              ---------- -------------
                 True     1.81±0.03ms 
                False     1.80±0.01ms 
              ========== =============

[ 55.21%] ··· io.csv.ReadCSVCategorical.time_convert_direct                                                                                       29.9±0.4ms
[ 56.25%] ··· io.csv.ReadCSVCategorical.time_convert_post                                                                                         46.6±0.9ms
[ 57.29%] ··· io.csv.ReadCSVComment.time_comment                                                                                                    24.8±2ms
[ 58.33%] ··· io.csv.ReadCSVConcatDatetime.time_read_csv                                                                                            30.7±2ms
[ 59.38%] ··· io.csv.ReadCSVConcatDatetimeBadDateValue.time_read_csv                                                                                      ok
[ 59.38%] ··· ================ ============
               bad_date_value              
              ---------------- ------------
                    nan         14.2±0.8ms 
                     0          11.7±0.2ms 
                     ''         12.0±0.3ms 
              ================ ============

[ 60.42%] ··· io.csv.ReadCSVDInferDatetimeFormat.time_read_csv                                                                                            ok
[ 60.42%] ··· ======================= ============= ============= =============
              --                                        format                 
              ----------------------- -----------------------------------------
               infer_datetime_format      custom       iso8601         ymd     
              ======================= ============= ============= =============
                        True           5.54±0.06ms   2.05±0.02ms    2.07±0.1ms 
                       False             101±4ms     1.66±0.05ms   1.55±0.01ms 
              ======================= ============= ============= =============

[ 61.46%] ··· io.csv.ReadCSVFloatPrecision.time_read_csv                                                                                                  ok
[ 61.46%] ··· ===== ============= ============= ================ ============= ============= ================
              --                                    decimal / float_precision                                
              ----- -----------------------------------------------------------------------------------------
               sep     . / None      . / high    . / round_trip     _ / None      _ / high    _ / round_trip 
              ===== ============= ============= ================ ============= ============= ================
                ,    1.69±0.04ms   1.64±0.04ms    2.96±0.07ms     1.83±0.08ms   1.82±0.04ms    1.87±0.06ms   
                ;    1.70±0.08ms   1.65±0.06ms    2.98±0.08ms     1.82±0.05ms   1.86±0.07ms     1.90±0.1ms   
              ===== ============= ============= ================ ============= ============= ================

[ 62.50%] ··· io.csv.ReadCSVFloatPrecision.time_read_csv_python_engine                                                                                    ok
[ 62.50%] ··· ===== ============= ============= ================ ============= ============= ================
              --                                    decimal / float_precision                                
              ----- -----------------------------------------------------------------------------------------
               sep     . / None      . / high    . / round_trip     _ / None      _ / high    _ / round_trip 
              ===== ============= ============= ================ ============= ============= ================
                ,    4.42±0.05ms   4.44±0.08ms    4.47±0.05ms     3.77±0.06ms   3.87±0.03ms    3.85±0.07ms   
                ;    4.42±0.02ms   4.45±0.08ms    4.42±0.02ms      3.98±0.1ms   3.82±0.05ms    3.82±0.05ms   
              ===== ============= ============= ================ ============= ============= ================

[ 63.54%] ··· io.csv.ReadCSVMemoryGrowth.mem_parser_chunks                                                                                                 0
[ 64.58%] ··· io.csv.ReadCSVParseDates.time_baseline                                                                                             1.70±0.01ms
[ 65.62%] ··· io.csv.ReadCSVParseDates.time_multiple_date                                                                                        2.02±0.02ms
[ 66.67%] ··· io.csv.ReadCSVParseSpecialDate.time_read_special_date                                                                                       ok
[ 66.67%] ··· ======= =============
               value               
              ------- -------------
                 mY     7.76±0.3ms 
                mdY    3.90±0.05ms 
                 hm    3.43±0.05ms 
              ======= =============

[ 67.71%] ··· io.csv.ReadCSVSkipRows.time_skipprows                                                                                                       ok
[ 67.71%] ··· ========== =============
               skiprows               
              ---------- -------------
                 None      17.1±0.1ms 
                10000     11.8±0.06ms 
              ========== =============

[ 68.75%] ··· io.csv.ReadCSVThousands.time_thousands                                                                                                      ok
[ 68.75%] ··· ===== ============= ============
              --            thousands         
              ----- --------------------------
               sep       None          ,      
              ===== ============= ============
                ,    15.8±0.09ms   15.1±0.3ms 
                |     15.8±0.1ms   15.5±0.4ms 
              ===== ============= ============

[ 69.79%] ··· io.csv.ReadUint64Integers.time_read_uint64                                                                                          3.14±0.2ms
[ 70.83%] ··· io.csv.ReadUint64Integers.time_read_uint64_na_values                                                                                5.54±0.3ms
[ 71.88%] ··· io.csv.ReadUint64Integers.time_read_uint64_neg_values                                                                               5.32±0.3ms
[ 72.92%] ··· io.csv.ToCSV.time_frame                                                                                                                     ok
[ 72.92%] ··· ======= ============
                kind              
              ------- ------------
                wide    149±2ms   
                long    215±3ms   
               mixed   26.2±0.2ms 
              ======= ============

[ 73.96%] ··· io.csv.ToCSVDatetime.time_frame_date_formatting                                                                                     12.0±0.1ms
[ 75.00%] ··· io.csv.ToCSVDatetimeBig.time_frame                                                                                                          ok
[ 75.00%] ··· ======== =============
                obs                 
              -------- -------------
                1000    7.22±0.08ms 
               10000     64.7±0.4ms 
               100000     647±6ms   
              ======== =============

[ 75.00%] · For pandas commit aefae55e <master> (round 2/2):
[ 75.00%] ·· Building for virtualenv-py3.7-Cython0.29.16-jinja2-matplotlib-numba-numexpr-numpy-odfpy-openpyxl-pytest-scipy-sqlalchemy-tables-xlrd-xlsxwriter-xlwt...
[ 75.00%] ·· Benchmarking virtualenv-py3.7-Cython0.29.16-jinja2-matplotlib-numba-numexpr-numpy-odfpy-openpyxl-pytest-scipy-sqlalchemy-tables-xlrd-xlsxwriter-xlwt
[ 76.04%] ··· io.csv.ParseDateComparison.time_read_csv_dayfirst                                                                                           ok
[ 76.04%] ··· ============= ============
               cache_dates              
              ------------- ------------
                  False      8.22±0.1ms 
                   True      3.88±0.2ms 
              ============= ============

[ 77.08%] ··· io.csv.ParseDateComparison.time_to_datetime_dayfirst                                                                                        ok
[ 77.08%] ··· ============= =============
               cache_dates               
              ------------- -------------
                  False       8.74±0.3ms 
                   True      3.89±0.09ms 
              ============= =============

[ 78.12%] ··· io.csv.ParseDateComparison.time_to_datetime_format_DD_MM_YYYY                                                                               ok
[ 78.12%] ··· ============= ============
               cache_dates              
              ------------- ------------
                  False       23.0±1ms  
                   True      3.89±0.1ms 
              ============= ============

[ 79.17%] ··· io.csv.ReadCSVCachedParseDates.time_read_csv_cached                                                                                         ok
[ 79.17%] ··· ========== =============
               do_cache               
              ---------- -------------
                 True     1.88±0.05ms 
                False     1.83±0.03ms 
              ========== =============

[ 80.21%] ··· io.csv.ReadCSVCategorical.time_convert_direct                                                                                       30.4±0.6ms
[ 81.25%] ··· io.csv.ReadCSVCategorical.time_convert_post                                                                                         46.8±0.4ms
[ 82.29%] ··· io.csv.ReadCSVComment.time_comment                                                                                                  25.4±0.4ms
[ 83.33%] ··· io.csv.ReadCSVConcatDatetime.time_read_csv                                                                                            32.0±1ms
[ 84.38%] ··· io.csv.ReadCSVConcatDatetimeBadDateValue.time_read_csv                                                                                      ok
[ 84.38%] ··· ================ ============
               bad_date_value              
              ---------------- ------------
                    nan         14.5±0.6ms 
                     0          11.9±0.3ms 
                     ''         13.4±0.7ms 
              ================ ============

[ 85.42%] ··· io.csv.ReadCSVDInferDatetimeFormat.time_read_csv                                                                                            ok
[ 85.42%] ··· ======================= ============= ============= =============
              --                                        format                 
              ----------------------- -----------------------------------------
               infer_datetime_format      custom       iso8601         ymd     
              ======================= ============= ============= =============
                        True           5.54±0.08ms   2.06±0.03ms   2.09±0.02ms 
                       False             102±6ms     1.65±0.03ms   1.61±0.07ms 
              ======================= ============= ============= =============

[ 86.46%] ··· io.csv.ReadCSVFloatPrecision.time_read_csv                                                                                                  ok
[ 86.46%] ··· ===== ============= ============= ================ ============= ============= ================
              --                                    decimal / float_precision                                
              ----- -----------------------------------------------------------------------------------------
               sep     . / None      . / high    . / round_trip     _ / None      _ / high    _ / round_trip 
              ===== ============= ============= ================ ============= ============= ================
                ,    1.70±0.05ms   1.65±0.05ms    2.70±0.08ms     1.86±0.04ms   1.82±0.06ms    1.84±0.05ms   
                ;    1.73±0.04ms   1.68±0.07ms    2.74±0.06ms     1.83±0.03ms   1.83±0.05ms    1.86±0.06ms   
              ===== ============= ============= ================ ============= ============= ================

[ 87.50%] ··· io.csv.ReadCSVFloatPrecision.time_read_csv_python_engine                                                                                    ok
[ 87.50%] ··· ===== ============= ============= ================ ============ ============= ================
              --                                   decimal / float_precision                                
              ----- ----------------------------------------------------------------------------------------
               sep     . / None      . / high    . / round_trip    _ / None      _ / high    _ / round_trip 
              ===== ============= ============= ================ ============ ============= ================
                ,     4.52±0.1ms    4.51±0.1ms     4.51±0.1ms     4.26±0.5ms   3.83±0.05ms    3.85±0.07ms   
                ;    4.37±0.07ms   4.42±0.03ms    4.42±0.08ms     3.83±0.1ms    3.98±0.2ms    3.86±0.06ms   
              ===== ============= ============= ================ ============ ============= ================

[ 88.54%] ··· io.csv.ReadCSVMemoryGrowth.mem_parser_chunks                                                                                                 0
[ 89.58%] ··· io.csv.ReadCSVParseDates.time_baseline                                                                                             1.70±0.02ms
[ 90.62%] ··· io.csv.ReadCSVParseDates.time_multiple_date                                                                                        2.06±0.03ms
[ 91.67%] ··· io.csv.ReadCSVParseSpecialDate.time_read_special_date                                                                                       ok
[ 91.67%] ··· ======= =============
               value               
              ------- -------------
                 mY     7.92±0.4ms 
                mdY    3.88±0.04ms 
                 hm    3.41±0.06ms 
              ======= =============

[ 92.71%] ··· io.csv.ReadCSVSkipRows.time_skipprows                                                                                                       ok
[ 92.71%] ··· ========== ============
               skiprows              
              ---------- ------------
                 None     17.1±0.2ms 
                10000     11.9±0.1ms 
              ========== ============

[ 93.75%] ··· io.csv.ReadCSVThousands.time_thousands                                                                                                      ok
[ 93.75%] ··· ===== ============= ============
              --            thousands         
              ----- --------------------------
               sep       None          ,      
              ===== ============= ============
                ,    15.9±0.06ms   15.3±0.3ms 
                |     16.0±0.2ms   15.7±0.3ms 
              ===== ============= ============

[ 94.79%] ··· io.csv.ReadUint64Integers.time_read_uint64                                                                                          3.13±0.3ms
[ 95.83%] ··· io.csv.ReadUint64Integers.time_read_uint64_na_values                                                                                5.54±0.3ms
[ 96.88%] ··· io.csv.ReadUint64Integers.time_read_uint64_neg_values                                                                               5.35±0.2ms
[ 97.92%] ··· io.csv.ToCSV.time_frame                                                                                                                     ok
[ 97.92%] ··· ======= ============
                kind              
              ------- ------------
                wide   146±0.7ms  
                long   212±0.4ms  
               mixed   25.9±0.1ms 
              ======= ============

[ 98.96%] ··· io.csv.ToCSVDatetime.time_frame_date_formatting                                                                                     11.9±0.1ms
[100.00%] ··· io.csv.ToCSVDatetimeBig.time_frame                                                                                                          ok
[100.00%] ··· ======== =============
                obs                 
              -------- -------------
                1000    7.25±0.05ms 
               10000     64.6±0.4ms 
               100000    690±100ms  
              ======== =============


BENCHMARKS NOT SIGNIFICANTLY CHANGED.

```
</details>

`BENCHMARKS NOT SIGNIFICANTLY CHANGED.` thanks @ales-erjavec 