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 33dddac0)
Co-authored-by: Anselm Kruis <a.kruis@science-computing.de>