BUG: reading windows utf8 filenames in py3.6
- [x] closes #15086 properly
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Test that was added in prior fix was failing on my machine, where my locale was set to `CP-1251` (which apparently cannot handle some unicode symbols in the test even when using MBCS, as MBCS encoding is locale-dependent).

This PR fixes the issue removing the workaround by switching to converting UTF8 to UTF16 and using Win32 Unicode file API (much like Python >= 3.6 does internally).

P.S. This as a bonus adds possibility to use `\\?\C\long\file\name` addressing if desired to overcome 260 symbols limitation on path length.

> Test that was added in prior fix was failing on my machine, where my locale was set to `CP-1251`

is there an open issue about this?

cc @gfyoung  I didn't open an issue when I faced that, instead I have fixed the code.
Should I file an issue before the code could be accepted? It normally helps to create a code sample which can be used as the test and for others to recreate the problem. The test that failed was the same test that was introduced in #24758, it just didn't pass on my machine, most likely due to different locale set up. My fix is locale-independent, so it should pass on any machine with any locale set. so i guess that `test_filename_with_special_chars` should be parametrised over different locales. would monkeypatch allow this here? My diggings show that when Python faces a request to convert from Unicode to bytestring using `mbcs` encoding on Windows it uses `CP_ACP` as codepage for `WideCharToMultiByte` argument (that is, default ANSI code page).

According to https://stackoverflow.com/a/9039278 there is no programmatic way to change `CP_ACP` value, as this is a system-wide setting controlled via Region settings in Windows Control Panel.

So, in short, the answer to this question:


> would monkeypatch allow this here?

is "no" :(

What _does_ seem to be possible is to change the test so it would fail _both_ for `1252` and `1251` codepages unless fixed as I do here by using a superset of characters some of which are not representable in 1251 and some in 1252.

Note that it would require some dancing in the test to not break Python < 3.6, though. > is there an open issue about this?

Not that I'm aware of.

> What does seem to be possible is to change the test so it would fail both for 1252 and 1251 codepages unless fixed as I do here by using a superset of characters some of which are not representable in 1251 and some in 1252.

I would just add a new test actually.

Second, if you could post the output that you were getting prior to this fix for that test, that would be good for everyone's reference.

Finally, if you could add a whatsnew note for `0.25.0`.  Even though it's fixing an already-closed issue, you're adding a more robust fix.  Otherwise, so far so good! > I would just add a new test actually.

I'm not sure what second test would add here, what I'm thinking is simply crafting a string that isn't representable in neither `1251` nor `1252` (my current stab at it is `sé-es-vé-sй.csv` - it cannot be encoded neither as 1251 nor as 1252 bytestrings). The only quirk that remains there is that I'd probably have to disable the test for Python < 3.6 (otherwise it would forever fail there on strange locales - as prior to 3.6 I cannot be sure that what this function gets as an input parameter is a UTF-8-encoded string).

> Second, if you could post the output that you were getting prior to this fix for that test, that would be good for everyone's reference.

Failure without my patch is easy:

```
________________ test_filename_with_special_chars[c_high] ____________________________________

all_parsers = <pandas.tests.io.parser.conftest.CParserHighMemory object at 0x000002CA11F92320>

    def test_filename_with_special_chars(all_parsers):
        # see gh-15086.
        parser = all_parsers
        df = DataFrame({"a": [1, 2, 3]})

        with tm.ensure_clean("sé-es-vé.csv") as path:
            df.to_csv(path, index=False)

>           result = parser.read_csv(path)

pandas\tests\io\parser\test_common.py:1915:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
pandas\tests\io\parser\conftest.py:22: in read_csv
    return read_csv(*args, **kwargs)
pandas\io\parsers.py:702: in parser_f
    return _read(filepath_or_buffer, kwds)
pandas\io\parsers.py:429: in _read
    parser = TextFileReader(filepath_or_buffer, **kwds)
pandas\io\parsers.py:895: in __init__
    self._make_engine(self.engine)
pandas\io\parsers.py:1122: in _make_engine
    self._engine = CParserWrapper(self.f, **self.options)
pandas\io\parsers.py:1853: in __init__
    self._reader = parsers.TextReader(src, **kwds)
pandas\_libs\parsers.pyx:388: in pandas._libs.parsers.TextReader.__cinit__
    self._setup_parser_source(source)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

>   source = source.encode(encoding)
E   UnicodeEncodeError: 'mbcs' codec can't encode characters in position 0--1: invalid character

pandas\_libs\parsers.pyx:687: UnicodeEncodeError
```

> Finally, if you could add a whatsnew note for `0.25.0`

Will do tomorrow. > The only quirk that remains there is that I'd probably have to disable the test for Python < 3.6 (otherwise it would forever fail there on strange locales - as prior to 3.6 I cannot be sure that what this function gets as an input parameter is a UTF-8-encoded string)

No worries.  Feel free to skip the test for Python < 3.6 (we're already in talks about phasing out 3.5 support once we finish with 2.7). pls don’t skip for 3.5  > pls don’t skip for 3.5

@jreback : @vnlitvin fix relies on leveraging Python 3.6+ internal functionality that can't be replicated in Python 3.5.  That is why I suggested he add a **new test** (instead of modifying the existing one) that can then be skipped.

The changes he has made are otherwise compatible with the rest of our tests. ah I c. carry on then. > the existing one

The problem with existing one is that it would fail on Python 3.5 with e.g. 1251 (Cyrillic) locale set.
What can I add is I can skip the test on non-1252 locales for Python < 3.6 (though it seems that the test is kinda useless for < 3.6 - it basically tests that Windows `open()` function works - doesn't seem to be a job for a Pandas test). > What can I add is I can skip the test on non-1252 locales for Python < 3.6

Yep, that sounds like a plan.  Go for it! Hello @vnlitvin! 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 2019-03-20 10:01:09 UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25769?src=pr&el=h1) Report
> Merging [#25769](https://codecov.io/gh/pandas-dev/pandas/pull/25769?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e8d951d2edef5df2cc7bf60e90f345fd5fd18a6b?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25769/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25769?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25769   +/-   ##
=======================================
  Coverage   91.25%   91.25%           
=======================================
  Files         172      172           
  Lines       52965    52965           
=======================================
  Hits        48335    48335           
  Misses       4630     4630
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.83% <ø> (ø)` | :arrow_up: |
| #single | `41.74% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25769?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25769?src=pr&el=footer). Last update [e8d951d...cf2e062](https://codecov.io/gh/pandas-dev/pandas/pull/25769?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25769?src=pr&el=h1) Report
> Merging [#25769](https://codecov.io/gh/pandas-dev/pandas/pull/25769?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/46639512c06300a9844ea27f90167d5648c9b93a?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25769/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25769?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25769   +/-   ##
=======================================
  Coverage   91.26%   91.26%           
=======================================
  Files         173      173           
  Lines       52982    52982           
=======================================
  Hits        48356    48356           
  Misses       4626     4626
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.83% <ø> (ø)` | :arrow_up: |
| #single | `41.75% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25769?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25769?src=pr&el=footer). Last update [4663951...6095ea3](https://codecov.io/gh/pandas-dev/pandas/pull/25769?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Note for reviewers: it seems that testing is somehow broken in the upstream, as I didn't change anything related to `numpy` or `pytables`, but they now fail. It seems that the image used for testing has changed. @jreback should I mark as resolved the conversations I fixed? @vnlitvin yes you can resolve things.

we already know that the CI is failing. I will ask you to rebase at some point. can you merge master > can you merge master

I've rebased on latest master, so commit history will be clean. thanks @vnlitvin 