pyproject.toml: Set disallow_untyped_defs = true

We pass this check.

pyproject.toml

@@ -49,3 +49,4 @@ check_untyped_defs = true
 disallow_incomplete_defs = true
 disallow_untyped_calls = true
 disallow_untyped_decorators = true
+disallow_untyped_defs = true