TST/REF: io/parser/(test_dtypes.py, test_usecols.py)
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

#38370 adds a pyarrow engine to the csv reader. Only a fraction of the io/parser tests pass when pyarrow is used and the rest has to be xfailed/skipped, resulting in a large diff on the PR.

xref https://github.com/pandas-dev/pandas/pull/38370#discussion_r539579072 suggests reorganizing the tests into classes so groups of tests can be xfailed with a single mark.

I'm grouping the tests logically (not based on whether or not they pass with pyarrow) but merging this this _will_ reduce the diff in #38370 substantively.

Likely I will submit a follow-on with some further reorg. I'm happy to push that to this PR if that's preferred, though.

Verifying that total number of tests is unchanged:
```
(pandas-dev) andrewwieteska@Andrews-MacBook-Pro pandas % pytest  pandas/tests/io/parser/test_dtypes.py  pandas/tests/io/parser/test_usecols.py       
========================================================================= test session starts =========================================================================
platform darwin -- Python 3.8.6, pytest-6.1.2, py-1.9.0, pluggy-0.13.1
rootdir: /Users/andrewwieteska/repos/pandas, configfile: setup.cfg
plugins: forked-1.2.0, xdist-2.1.0, cov-2.10.1, asyncio-0.14.0, hypothesis-5.41.2, instafail-0.4.1
collected 372 items                                                                                                                                                   

pandas/tests/io/parser/test_dtypes.py ......................................................................................................................... [ 32%]
............................................................................................                                                                    [ 57%]
pandas/tests/io/parser/test_usecols.py ........................................................................................................................ [ 89%]
.......................................                                                                                                                         [100%]

======================================================================== 372 passed in 32.49s =========================================================================
```
versus on master:
```
(pandas-dev) andrewwieteska@Andrews-MacBook-Pro pandas % pytest  pandas/tests/io/parser/test_dtypes.py  pandas/tests/io/parser/test_usecols.py
========================================================================= test session starts =========================================================================
platform darwin -- Python 3.8.6, pytest-6.1.2, py-1.9.0, pluggy-0.13.1
rootdir: /Users/andrewwieteska/repos/pandas, configfile: setup.cfg
plugins: forked-1.2.0, xdist-2.1.0, cov-2.10.1, asyncio-0.14.0, hypothesis-5.41.2, instafail-0.4.1
collected 372 items                                                                                                                                                   

pandas/tests/io/parser/test_dtypes.py ......................................................................................................................... [ 32%]
............................................................................................                                                                    [ 57%]
pandas/tests/io/parser/test_usecols.py ........................................................................................................................ [ 89%]
.......................................                                                                                                                         [100%]

======================================================================== 372 passed in 32.90s =========================================================================
```
instead of using classes, can you just create / rename more module files? its the same effect. 

cc @gfyoung  100% agree with @jreback feedback.  Functions are more idiomatic with `pytest`.

A module will achieve what you are trying to do with these classes.  Otherwise, a custom fixture will do. Sgtm - will put these into modules looks like something is failing, merge master. pls confrm same number of tests on master as on here.  > looks like something is failing

I used the same name for a base file between two folders - fixed now
 This patch:
```
(pandas-dev) andrewwieteska@Andrews-MacBook-Pro pandas % pytest pandas/tests/io/parser/dtypes pandas/tests/io/parser/usecols  
===================================================== test session starts =====================================================
platform darwin -- Python 3.8.6, pytest-6.1.2, py-1.9.0, pluggy-0.13.1
rootdir: /Users/andrewwieteska/repos/pandas, configfile: setup.cfg
plugins: forked-1.2.0, xdist-2.1.0, cov-2.10.1, asyncio-0.14.0, hypothesis-5.41.2, instafail-0.4.1
collected 372 items                                                                                                           

pandas/tests/io/parser/dtypes/test_categorical.py ..................................................................... [ 18%]
.....................                                                                                                   [ 24%]
pandas/tests/io/parser/dtypes/test_dtypes_basic.py .................................................................... [ 42%]
.......                                                                                                                 [ 44%]
pandas/tests/io/parser/dtypes/test_empty.py ................................................                            [ 57%]
pandas/tests/io/parser/usecols/test_parse_dates.py ...........................                                          [ 64%]
pandas/tests/io/parser/usecols/test_strings.py ..................                                                       [ 69%]
pandas/tests/io/parser/usecols/test_usecols_basic.py .................................................................. [ 87%]
................................................                                                                        [100%]

==================================================== 372 passed in 30.93s =====================================================
```
On master:
```
(pandas-dev) andrewwieteska@Andrews-MacBook-Pro pandas % pytest pandas/tests/io/parser/test_dtypes.py pandas/tests/io/parser/test_usecols.py 
===================================================== test session starts =====================================================
platform darwin -- Python 3.8.6, pytest-6.1.2, py-1.9.0, pluggy-0.13.1
rootdir: /Users/andrewwieteska/repos/pandas, configfile: setup.cfg
plugins: forked-1.2.0, xdist-2.1.0, cov-2.10.1, asyncio-0.14.0, hypothesis-5.41.2, instafail-0.4.1
collected 372 items                                                                                                           

pandas/tests/io/parser/test_dtypes.py ................................................................................. [ 21%]
....................................................................................................................... [ 53%]
.............                                                                                                           [ 57%]
pandas/tests/io/parser/test_usecols.py ................................................................................ [ 78%]
...............................................................................                                         [100%]

==================================================== 372 passed in 28.89s =====================================================
``` Green thanks