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>

gclient_utils.py

@@ -17,12 +17,6 @@ import operator
 import os
 import pipes
 import platform
-
-try:
-  import Queue as queue
-except ImportError:  # For Py3 compatibility
-  import queue
-
 import re
 import stat
 import subprocess
@@ -30,18 +24,16 @@ import sys
 import tempfile
 import threading
 import time
-
-try:
-  import urlparse
-except ImportError:  # For Py3 compatibility
-  import urllib.parse as urlparse
-
 import subprocess2
 
 if sys.version_info.major == 2:
   from cStringIO import StringIO
+  import Queue as queue
+  import urlparse
 else:
   from io import StringIO
+  import queue
+  import urllib.parse as urlparse
 
 
 RETRY_MAX = 3