bpo-39899: Make pathlib use os.path.expanduser() to expand home directories

[barneygale]

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.

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 on Windows.

https://bugs.python.org/issue39899

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[pitrou]

I see no problem with this personally but I would like advice from our Windows experts, @zooba and @pfmoore, especially on the os.path.expanduser changes.

[barneygale]

Updated to use os.fsdecode() per review comments. Thanks for your help so far @pitrou.

@pfmoore@zooba flagging this with you again per pitrou's recommendation!

[zooba]

I think this one is okay, but I need to spend a bit more time reading through it (it's pretty late here now and I'm too tired).

[barneygale]

Yep understood! If it helps, the rough process here is:

1. Bring os.path.expanduser() in line with pathlib's home directory resolution. They were already almost identical, but pathlib was slightly stricter in the rare scenario:

• We're on Windows, and
• USERNAME is set, and
• It doesn't match the last component of the current user's home directory, and
• We're querying for another user's home directory

2. Delete pathlib's implementation in the name of DRY.

[barneygale]

Rebased to fix conflicts