[3.12] GH-104947: Make pathlib.PureWindowsPath comparisons consistent across platforms (GH-104948) (GH-104990)

Use `str.lower()` rather than `ntpath.normcase()` to normalize case of
Windows paths. This restores behaviour from Python 3.11.

(cherry picked from commit ad0be361c9922a918c7c3eaf83e1d8f2b019279c)

Co-authored-by: Barney Gale <barney.gale@gmail.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>