Add async def wait to EventQueue objects

[jepler]

In conjunction with adafruit/Adafruit_CircuitPython_asyncio#25 this allows keypad-using code to await the presence of a new event:

async def aget(scanner):
    await scanner.events.wait()
    return scanner.events.get()

async def keeb(pin):
    with keypad.Keys((pin,), value_when_pressed=False) as kb:
        while True:
            print(await aget(kb))

Aside from other issues (it'd be nicer to not need the aget function at all but be able to await kb.events.aget(), it's also a bit weird for built in functionality to rely on code in an .py module.

[jepler]

I think #6712 is probably a better way forward.