bpo-42146: Fix memory leak in subprocess.Popen() in case of uid/gid overflow (GH...

bpo-42146: Fix memory leak in subprocess.Popen() in case of uid/gid overflow (GH-22966)

Fix memory leak in subprocess.Popen() in case of uid/gid overflow

Also add a test that would catch this leak with `--huntrleaks`.

Alas, the test for `extra_groups` also exposes an inconsistency
in our error reporting: we use a custom ValueError for `extra_groups`,
but propagate OverflowError for `user` and `group`.