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/handlers/oidc_handler.py

@@ -149,6 +149,9 @@ class OidcHandler:
         Args:
             request: the incoming request from the browser.
         """
+        # This will always be set by the time Twisted calls us.
+        assert request.args is not None
+
         # The provider might redirect with an error.
         # In that case, just display it as-is.
         if b"error" in request.args: