implement sniffio using sys.get_asyncgen_hooks

[graingert]

this works for trio curio and asyncio already. All twisted needs to do is implement set_asyncgen_hooks

def sniffio(_get_asyncgen_hooks=sys._get_asyncgen_hooks):
    hooks = _get_asyncgen_hooks()
    finalizer = hooks.finalizer
    if finalizer is not None:
        return finalizer.__module__.partition(".")[0]
    firstiter = hooks.firstiter
    if firstiter is not None:
        return firstiter.__module__.partition(".")[0]

trio-asyncio can twiddle with hooks.finalizer.__module__ to go back and forth between asyncio and trio