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/client/v2_alpha/sync.py

@@ -91,6 +91,9 @@ class SyncRestServlet(RestServlet):
         self._event_serializer = hs.get_event_client_serializer()
 
     async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
+        # This will always be set by the time Twisted calls us.
+        assert request.args is not None
+
         if b"from" in request.args:
             # /events used to use 'from', but /sync uses 'since'.
             # Lets be helpful and whine if we see a 'from'.