[3.6] bpo-30028: make test.support.temp_cwd() fork-safe (GH-1066) (GH-5826)

[3.6] bpo-30028: make test.support.temp_cwd() fork-safe (GH-1066) (GH-5826)

Make test.support.temp_cwd() fork-safe. The context manager test.support.temp_cwd() no longer removes the temporary directory when executing in a process other than the parent it entered from.
If a forked child exits the context manager it won't do the cleanup..
(cherry picked from commit 33dddac00ba8d9b72cf21b8698504077eb3c23ad)

Co-authored-by: Anselm Kruis <a.kruis@science-computing.de>