pyproject.toml: Set disallow_untyped_decorators = true

We already pass this, so might as well enable it.

pyproject.toml

@@ -47,3 +47,4 @@ line-length=100
 [tool.mypy]
 check_untyped_defs = true
 disallow_untyped_calls = true
+disallow_untyped_decorators = true