import mock from third_party explicitly

Stop relying on third_party/ leaking into sys.path all the time.

BUG=b:187794934
TEST=CQ passes

Change-Id: If233859063e6d2a1f2fdaa851febcbe8f536896a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2896053
Tested-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
diff --git a/scripts/cbuildbot_launch_unittest.py b/scripts/cbuildbot_launch_unittest.py
index 9bb3db1..8a9a6ff 100644
--- a/scripts/cbuildbot_launch_unittest.py
+++ b/scripts/cbuildbot_launch_unittest.py
@@ -11,8 +11,6 @@
 import sys
 import time
 
-import mock
-
 from chromite.cbuildbot import commands
 from chromite.cbuildbot import repository
 from chromite.lib import build_summary
@@ -22,6 +20,7 @@
 from chromite.lib import osutils
 from chromite.lib import timeout_util
 from chromite.scripts import cbuildbot_launch
+from chromite.third_party import mock
 
 
 assert sys.version_info >= (3, 6), 'This module requires Python 3.6+'