The only WIN32 specific behavior the drive or UNC prefix is skipped
(except for it's slash), allowing WIN32 & Unix paths to share the same
code-path.
Note that this changes behavior for WIN32 when there are too many parent
directories, where ".." that move before the root directory are removed
instead of being left in the path. e.g:
`C:\a\..\..\b` resolved to `C:\..\b` whereas now the result is `C:\b`.
This matches Python's `os.path.normpath`.
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