bpo-40701: tempfile mixes str and bytes in an inconsistent manner (GH-20442)

The case of tempfile.tempdir variable being bytes is now handled consistently.
The getters return the right type and no more error of mixing str and bytes unless explicitly caused by the user.

Adds a regression test.

Expands the documentation to clarify the behavior.

Co-authored-by: Eric L <ewl+git@lavar.de>
Co-authored-by: Gregory P. Smith <greg@krypto.org>