depot_tools: Use mock from vpython (or unittest.mock) instead of third_party/mock

Change-Id: I3a188b34ae5f62649108afe08fe0e389a408c2ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1947933
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Anthony Polito <apolito@google.com>
diff --git a/tests/gclient_scm_test.py b/tests/gclient_scm_test.py
index 6fb0b76..b6b3cdf 100755
--- a/tests/gclient_scm_test.py
+++ b/tests/gclient_scm_test.py
@@ -22,12 +22,13 @@
 
 if sys.version_info.major == 2:
   from cStringIO import StringIO
+  import mock
 else:
   from io import StringIO
+  from unittest import mock
 
 sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 
-from third_party import mock
 from testing_support import fake_repos
 from testing_support import test_case_utils