bpo-25625: add contextlib.chdir (GH-28271)

Added non parallel-safe :func:`~contextlib.chdir` context manager to change
the current working directory and then restore it on exit. Simple wrapper
around :func:`~os.chdir`.

Signed-off-by: Filipe Laíns <lains@riseup.net>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>