Use the Python mock implementation without deprecated asyncio calls.

--- tests/unit/test_bucket.py.orig
+++ tests/unit/test_bucket.py
@@ -15,7 +15,7 @@
 import datetime
 import unittest
 
-import mock
+from unittest import mock
 import pytest
 
 from google.cloud.storage._helpers import _NOW, _UTC, _get_default_storage_base_url
