bpo-39899: os.path.expanduser(): don't guess other Windows users' home directories...

bpo-39899: os.path.expanduser(): don't guess other Windows users' home directories if the basename of the current user's home directory doesn't match their username. (GH-18841)

This makes `ntpath.expanduser()` match `pathlib.Path.expanduser()` in this regard, and is more in line with `posixpath.expanduser()`'s cautious approach.

Also remove the near-duplicate implementation of `expanduser()` in pathlib, and by doing so fix a bug where KeyError could be raised when expanding another user's home directory.