This is a slight variation on #6056, making uasyncio.Event().set() safe to call from a soft-scheduled callback (with micropython.schedule()).

This version does not introduce socket.socketpair() for the signalling, but rather hides this in the poll object as a more general "notifier" object. So you can now do this (independent to asyncio):

import uselect
poll = uselect.poll()

# pass None to register a special notifier entity; returns the notifier
# on unix the notifier is a socketpair, but it could be anything on other ports
notifier = poll.register(None, uselect.POLLIN)

def callback():
    # set the notifier, will be irq/signal/thread safe
    # this will wake any poll.poll() sleeps
    notifier.set()

# sleep until an event
for ev, s in poll.poll():
    if s is notifier:
        # got woken by the notifier, clear it
        s.clear()

This abstracts out a lot of the detail of poll and wake-up behaviour and should allow a bare-metal port more flexibility in implementing the notifier (eg with a semaphore).

Apart from the addition of micropython.scheduler_lock()/micropython.scheduler_unlock(), the only public facing API behaviour that is added here is the ability to call Event.set() from a scheduled callback. This would hopefully allow room for the underlying poll implementation to be completely changed (eg made more efficient) without the user noticing.