Fix twisted trunk mypy errors (#14012)

changelog.d/14012.misc

+Fix type annotations to be compatible with new annotations in development versions of twisted.

synapse/handlers/cas.py

@@ -130,6 +130,9 @@ class CasHandler:
         except PartialDownloadError as pde:
             # Twisted raises this error if the connection is closed,
             # even if that's being used old-http style to signal end-of-data
+            # Assertion is for mypy's benefit. Error.response is Optional[bytes],
+            # but a PartialDownloadError should always have a non-None response.
+            assert pde.response is not None
             body = pde.response
         except HttpResponseException as e:
             description = (

synapse/handlers/ui_auth/checkers.py

@@ -119,6 +119,9 @@ class RecaptchaAuthChecker(UserInteractiveAuthChecker):
         except PartialDownloadError as pde:
             # Twisted is silly
             data = pde.response
+            # For mypy's benefit. A general Error.response is Optional[bytes], but
+            # a PartialDownloadError.response should be bytes AFAICS.
+            assert data is not None
             resp_body = json_decoder.decode(data.decode("utf-8"))
 
         if "success" in resp_body: