bpo-39010: Fix errors logged on proactor loop restart (GH-22017) (#22035)

bpo-39010: Fix errors logged on proactor loop restart (GH-22017) (#22035)

Stopping and restarting a proactor event loop on windows can lead to
spurious errors logged (ConnectionResetError while reading from the
self pipe). This fixes the issue by ensuring that we don't attempt
to start multiple copies of the self-pipe reading loop.
(cherry picked from commit ea5a6363c3f8cc90b7c0cc573922b10f296073b6)

Co-authored-by: Ben Darnell <ben@bendarnell.com>
Co-authored-by: Ben Darnell <ben@bendarnell.com>