[3.6] bpo-30038: fix race condition in signal delivery + wakeup fd (#1082)

[vstinner]

Before, it was possible to get the following sequence of
events (especially on Windows, where the C-level signal handler for
SIGINT is run in a separate thread):

• SIGINT arrives
• trip_signal is called
• trip_signal writes to the wakeup fd
• the main thread wakes up from select()-or-equivalent
• the main thread checks for pending signals, but doesn't see any
• the main thread drains the wakeup fd
• the main thread goes back to sleep
• trip_signal sets is_tripped=1 and calls Py_AddPendingCall to notify
  the main thread the it should run the Python-level signal handler
• the main thread doesn't notice because it's asleep

This has been causing repeated failures in the Trio test suite:
python-trio/trio#119
(cherry picked from commit 4ae0149)