gh-115892: Fix ntpath.splitext() with UNC paths

[serhiy-storchaka]

os.path.splitext() on Windows no longer splits an "extension" from the server or share name in the UNC path.

• Issue: os.path.splitext() can split UNC drive on Windows #115892

[zooba]

As I posted on the issue, I don't particularly want splitext to depend on the start of the string (or on the entire string, if you want to view it that way). If the caller is splitext-ing something that isn't a path to a file, they'll get invalid results (e.g. if it's a directory). I don't think it's worth changing from an implementation that only considers the end of the string.

(I post this half to remind myself next time I come and look at the PR, because I nearly just hit "merge" before remembering what this was about 😆 )