fix(python): Fix handling of TextIOWrapper in write_csv

[ruihe774]

Fixes "write to closed file" in write_csv:

>>> df.write_csv(open("test.csv", "w"))
OSError: write to closed file

[codecov]

Codecov Report

Attention: Patch coverage is 79.16667% with 10 lines in your changes missing coverage. Please review.

Project coverage is 80.69%. Comparing base (3a3600c) to head (43eedd2).
Report is 5 commits behind head on main.

Files	Patch %	Lines
py-polars/src/file.rs	78.72%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17328      +/-   ##
==========================================
- Coverage   80.72%   80.69%   -0.03%     
==========================================
  Files        1475     1476       +1     
  Lines      193388   193497     +109     
  Branches     2760     2759       -1     
==========================================
+ Hits       156117   156151      +34     
- Misses      36761    36836      +75     
  Partials      510      510              

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[ritchie46]

Thanks, can you add a test that showed the issue before this fix?

[ruihe774]

Thanks, can you add a test that showed the issue before this fix?

Added

[ruihe774]

It is not necessary to extract .buffer from TextIOWrapper after #17315 - TextIOWrapper is also handled. We can simply remove these lines.

[ruihe774]

I'm sorry that there was a bug in #17315, namely the handling of TextIOWrapper in Windows and with non-UTF8 encoding. I've fixed it in 784241c.

Now TextIOWrapper with non-UTF8 encoding is denied to avoid incorrect encoding. Previously write_csv does not check that.

[ruihe774]

Because the requirement of encoding has become more strict, PYTHONUTF8=1 is added to CI to fix tests on Windows. UTF-8 mode will become the default with PEP 686.