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

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:
  https://github.com/python-trio/trio/issues/119