refactor(python-generator): rename misnamed testing function

Renamed 'test_PythonPackageNamespace___getitem__' to
'test_PythonPackageNamespace_add'.

tests/unit/core/python/namespaces/test_PythonPackageNamespace.py

@@ -311,7 +311,7 @@ def test_PythonPackageNamespace_string_representation(namespace, expected):
         ),
     ],
 )
-def test_PythonPackageNamespace___getitem__(
+def test_PythonPackageNamespace_add(
     namespace,
     new_namespace,
     expected,