Make .sleep(..) return a coroutine

[erikjohnston]

This helps ensure that mypy can catch places where we don't await on it, like in #18763.

[MadLittleMods]

It looks like this other spot doesn't have types and is in contrib/ so we don't necessarily need to change it but we could also align

synapse/contrib/cmdclient/http.py

Lines 178 to 181 in 7ed5566

	def sleep(self, seconds):
	d = defer.Deferred()
	reactor.callLater(seconds, d.callback, seconds)
	return d

[erikjohnston]

Oh, mmm good spot. Looking at it though I'm disinclined to try and move it to async/await as it still users generators and @defer.inlineCallbacks everywhere else. If anything I'd be inclined to remove the cmdclient package entirely