bpo-33540: Add block_on_close attr to socketserver (GH-6911)


Add a new block_on_close class attribute to ForkingMixIn and
ThreadingMixIn classes of socketserver to opt-in for pre-3.7 behaviour.
(cherry picked from commit 453bd0bc65b7ea6a18c43da69143ab10d54c0a35)

Co-authored-by: Victor Stinner <vstinner@redhat.com>
4 files changed