Enable ThreadSafeFlag

[jepler]

This didn't work since CircuitPython names the module io, not uio.

This extension to asyncio is not compatible with standard Python. However (like asyncio.sleep_ms), is a useful extension.

One thing about it is that the core can use ThreadSafeFlag to enable use like this:

async def keyboard_task(pin):
    with keypad.Keys((pin,), value_when_pressed=False) as kb:
        while True:
            await kb.events.wait()
            print(kb.events.get())

Other changes are needed in the core before this works, and it still feels a bit of a hack. It's too bad you can't just write

print(await kb.events.aget())

but I still didn't find a way for the wait to return a useful value. A Python-coded wrapper function such as

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

so now the only slightly awkward await aget(kb) is needed.

[tannewt]

Why not put threadsafe flag in its own module? That way asyncio stays CPython-compatible.

[dhalbert]

Why not put threadsafe flag in its own module? That way asyncio stays CPython-compatible.

Related to #3. Also, I had an idea for an ancillary library:
From #7 (comment):

So any TaskGroup we add will not be quite compatible, and we probably want to add it in an ancillary library (adafruit_asyncio or asynciox or xasyncio or ??).