bpo-28724: Add methods send_fds and recv_fds to the socket module (GH-12889)

The socket module now has the socket.send_fds() and socket.recv.fds() functions.
Contributed by Joannah Nanjekye, Shinya Okano (original patch)
and Victor Stinner.

Co-Authored-By: Victor Stinner <vstinner@redhat.com>