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/git_cl_test.py b/tests/git_cl_test.py
index 150369c..e3f6f57 100755
--- a/tests/git_cl_test.py
+++ b/tests/git_cl_test.py
@@ -17,10 +17,16 @@
 import tempfile
 import unittest
 
+if sys.version_info.major == 2:
+  from StringIO 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 testing_support.auto_stub import TestCase
-from third_party import mock
 
 import metrics
 # We have to disable monitoring before importing git_cl.
@@ -32,11 +38,6 @@
 import git_footers
 import subprocess2
 
-if sys.version_info.major == 2:
-  from StringIO import StringIO
-else:
-  from io import StringIO
-
 
 def callError(code=1, cmd='', cwd='', stdout=b'', stderr=b''):
   return subprocess2.CalledProcessError(code, cmd, cwd, stdout, stderr)