Make it possible to use dmypy (#9692)

Running `dmypy run` will do a `mypy` check while spinning up a daemon
that makes rerunning `dmypy run` a lot faster.

`dmypy` doesn't support `follow_imports = silent` and has
`local_partial_types` enabled, so this PR enables those options and
fixes the issues that were newly raised. Note that `local_partial_types`
will be enabled by default in upcoming mypy releases.

synapse/rest/admin/rooms.py

@@ -390,6 +390,9 @@ class JoinRoomAliasServlet(ResolveRoomIdMixin, RestServlet):
     async def on_POST(
         self, request: SynapseRequest, room_identifier: str
     ) -> Tuple[int, JsonDict]:
+        # This will always be set by the time Twisted calls us.
+        assert request.args is not None
+
         requester = await self.auth.get_user_by_req(request)
         await assert_user_is_admin(self.auth, requester.user)