bpo-35017, socketserver: don't accept request after shutdown (GH-9952)

Prior to this revision, after the shutdown of a `BaseServer`,
the server accepted a last single request
if it was sent between the server socket polling
and the polling timeout.

This can be problematic for instance for a server restart
for which you do not want to interrupt the service,
by not closing the listening socket during the restart.
One request failed because of this behavior.

Note that only one request failed,
following requests were not accepted, as expected.