chore: updates imports to ensure correct type hint processing under Py3.8

Author: chalmerlowe

packages/pandas-gbq/pandas_gbq/core/resource_references.py

@@ -2,6 +2,8 @@
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
+from __future__ import annotations
+
 import dataclasses
 import re
 from typing import Union

packages/pandas-gbq/pandas_gbq/schema/pyarrow_to_bigquery.py

@@ -1,6 +1,8 @@
 # Copyright (c) 2023 pandas-gbq Authors All rights reserved.
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
+ 
+from __future__ import annotations
 
 from typing import Optional, cast