Import twisted stubs from sydent

[babolivier]

Currently we have to force mypy to allow calling untyped functions from a typed context, because we call functions from twisted that aren't typed (at least as of #11). Sydent has a bunch of twisted stubs: https://github.com/matrix-org/sydent/tree/main/stubs - we should copy them to this repo and use them instead.

However it's not entirely clear to me how to do that in a way that wouldn't duplicate code, i.e. in a way we could import them in Sydent from this library (i.e. we want to move the stubs from Sydent to here, rather than just copying them over).