pyproject.toml: Set disallow_untyped_decorators = true (MR 2464)

We pass this too, so might as well ensure we don't regress.

pyproject.toml

@@ -53,3 +53,4 @@ addopts = [
 [tool.mypy]
 check_untyped_defs = true
 disallow_incomplete_defs = true
+disallow_untyped_decorators = true