gh-85681: Make ntpath.abspath() platform independent

[serhiy-storchaka]

ntpath.relpath() returned different results on Windows and on other platforms:

>>> ntpath.relpath('foo ', 'foo')   # on Windows
'.'
>>> ntpath.relpath('foo ', 'foo')   # on other platforms
'..\\foo '

relpath() is based on abspath(), which is implemented with GetFullPathNameW() on Windows. It strips trailing dots and spaces from the last component of the path, and a single trailing dot from other components, unless it is preceded by a dot. The pure Python abspath(), used on other platforms, now does the same.

normpath() is not changed: it does not strip them on Windows either.

The rules are not documented, they were determined by testing nt._getfullpathname() on Windows 11. The added tests are run on all platforms, so they check both implementations.

• Issue: ntpath.relpath behaves differently on Windows with trailing spaces #85681

[zooba]

Can we clearly mark the new function as "best effort to imitate Windows behaviour, but not a specification"?

I want to avoid getting stuck in the future having to use our own implementation (with all the quirks) everywhere because someone looks at it and decides that it's more important to have cross-platform consistency than it is to have cross-process/native consistency. I'd rather have weird edge cases be different between Windows/Linux than between Python and other native Windows apps.

Just a comment in the code will do, maybe in the tests as well.

[zooba]

With that, this PR looks good to me.