gracedb: updated mock imports for python3

gracedb/alerts/tests/test_access.py

-import mock
+try:
+    from unittest import mock
+except ImportError:  # python < 3
+    import mock
 
 from django.conf import settings
 from django.contrib.auth.models import Group as AuthGroup

gracedb/alerts/tests/test_email.py

-import mock
+try:
+    from unittest import mock
+except ImportError:  # python < 3
+    import mock
 
 from django.test import override_settings
 

gracedb/alerts/tests/test_phone.py

-import mock
+try:
+    from unittest import mock
+except ImportError:  # python < 3
+    import mock
 
 from django.conf import settings
 from django.test import override_settings

gracedb/alerts/tests/test_views.py

-import mock
+try:
+    from unittest import mock
+except ImportError:  # python < 3
+    import mock
 import pytest
 
 from django.conf import settings

gracedb/api/tests/test_authentication.py

 from base64 import b64encode
-import mock
+try:
+    from unittest import mock
+except ImportError:  # python < 3
+    import mock
 
 from django.conf import settings
 from django.urls import reverse

gracedb/api/tests/test_throttling.py

-import mock
+try:
+    from unittest import mock
+except ImportError:  # python < 3
+    import mock
 
 from django.conf import settings
 from django.core.cache import caches

gracedb/api/tests/utils.py

 from copy import deepcopy
-import mock
+try:
+    from unittest import mock
+except ImportError:  # python < 3
+    import mock
 
 from django.conf import settings
 from django.core.cache import caches