GH-75586: Fix case where PATHEXT isn't applied to items in PATH (Windows)

[csm10495]

GH-75586 - Fix case where PATHEXT isn't applied to items in PATH.

The logic change here makes it so that if the command passed into shutil.which on windows, it will be verified against how it is passed in and against the various items in PATHEXT.

For example:

shutil.which('C:/windows/system32/cmd') 

will now return

'C:/windows/system32/cmd.EXE'

Instead of None.

If this looks good, I'd be happy to create a changelog entry, etc. Just feeling the water with this first commit.

[csm10495]

@eryksun I think i fixed everything. Check again. Thanks!

[csm10495]

@zooba updated and added a corresponding test.

[zooba]

Looks good, and the tests all patch the call to that API so we shouldn't be impacted if the test machine has it configured differently.

Once CI passes, and unless someone else speaks up, I'll merge. Thanks for all your work on this!

[csm10495]

Got a doctest failure... The error is odd.. maybe its transient? I'll push an empty commit to see if it helps.

Testing of doctests in the sources finished, look at the results in build/doctest/output.txt.
python: ./Include/internal/pycore_typeobject.h:75: _PyType_GetModuleState: Assertion `et->ht_module' failed.
Aborted (core dumped)
make[1]: *** [Makefile:49: build] Error 134
make[1]: Leaving directory '/home/runner/work/cpython/cpython/Doc'
Testing of doctests in the sources finished, look at the results in build/doctest/output.txt
make: *** [Makefile:127: doctest] Error 1
make: Leaving directory '/home/runner/work/cpython/cpython/Doc'
Error: Process completed with exit code 2.

[csm10495]

@eryksun added. Thanks.

[eryksun]

LGTM. Thank you, Charles.

[vstinner]

This change introduced a regression: #127001.