chore(python): use new unittest.mock from the standard library

tests/unit/test_flow.py

@@ -20,8 +20,8 @@
 import re
 import random
 import socket
+from unittest import mock
 
-import mock
 import pytest
 import requests
 import urllib

tests/unit/test_helpers.py

@@ -15,8 +15,8 @@
 import datetime
 import json
 import os
+from unittest import mock
 
-import mock
 import pytest
 
 from google.auth import external_account_authorized_user

tests/unit/test_interactive.py

@@ -13,8 +13,8 @@
 # limitations under the License.
 
 import socket
+from unittest import mock
 
-import mock
 import pytest
 
 

tests/unit/test_tool.py

@@ -16,10 +16,10 @@
 import json
 import os.path
 import tempfile
+from unittest import mock
 
 import click.testing
 import google.oauth2.credentials
-import mock
 import pytest
 
 import google_auth_oauthlib.flow