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)
 

synapse/rest/admin/users.py

@@ -833,6 +833,9 @@ class UserMediaRestServlet(RestServlet):
     async def on_GET(
         self, request: SynapseRequest, user_id: str
     ) -> Tuple[int, JsonDict]:
+        # This will always be set by the time Twisted calls us.
+        assert request.args is not None
+
         await assert_requester_is_admin(self.auth, request)
 
         if not self.is_mine(UserID.from_string(user_id)):

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'.

synapse/rest/media/v1/preview_url_resource.py

@@ -187,6 +187,8 @@ class PreviewUrlResource(DirectServeJsonResource):
         respond_with_json(request, 200, {}, send_cors=True)
 
     async def _async_render_GET(self, request: SynapseRequest) -> None:
+        # This will always be set by the time Twisted calls us.
+        assert request.args is not None
 
         # XXX: if get_user_by_req fails, what should we do in an async render?
         requester = await self.auth.get_user_by_req(request)

synapse/rest/synapse/client/pick_username.py

@@ -104,6 +104,9 @@ class AccountDetailsResource(DirectServeHtmlResource):
         respond_with_html(request, 200, html)
 
     async def _async_render_POST(self, request: SynapseRequest):
+        # This will always be set by the time Twisted calls us.
+        assert request.args is not None
+
         try:
             session_id = get_username_mapping_session_cookie_from_request(request)
         except SynapseError as e: