Fix PythonVirtualenvOperator templated_fields (#23559)

* Fix `PythonVirtualenvOperator` templated_fields
The `PythonVirtualenvOperator` templated_fields override `PythonOperator` templated_fields which caused functionality not to work as expected.
fixes: #23557

(cherry picked from commit 1657bd2)

airflow/operators/python.py

@@ -327,7 +327,8 @@ class PythonVirtualenvOperator(PythonOperator):
         processing templated fields, for examples ``['.sql', '.hql']``
     """
 
-    template_fields: Sequence[str] = ('requirements',)
+    template_fields: Sequence[str] = tuple({'requirements'} | set(PythonOperator.template_fields))
+
     template_ext: Sequence[str] = ('.txt',)
     BASE_SERIALIZABLE_CONTEXT_KEYS = {
         'ds',

tests/operators/test_python.py

@@ -868,6 +868,9 @@ def _run_as_operator(self, fn, python_version=sys.version_info[0], **kwargs):
         task.run(start_date=DEFAULT_DATE, end_date=DEFAULT_DATE)
         return task
 
+    def test_template_fields(self):
+        assert set(PythonOperator.template_fields).issubset(PythonVirtualenvOperator.template_fields)
+
     def test_add_dill(self):
         def f():
             """Ensure dill is correctly installed."""