This was due to a conflict between #11282, which changed mypy configuration, and #11228, a normal change.
@@ -0,0 +1 @@
+Require all files in synapse/ and tests/ to pass mypy unless specifically excluded.
@@ -363,6 +363,9 @@ ignore_missing_imports = True
[mypy-opentracing]
ignore_missing_imports = True
+[mypy-parameterized.*]
+ignore_missing_imports = True
+
[mypy-phonenumbers.*]