[autotest] Add Urllib3 package to external_packages.py

Some autotest servers don't have urllib3 installed.

BUG=None
TEST=pip uninstall urllib3; ./build_externals;
     inside python: import common, import urllib3,
     verified import succeeded.
DEPLOY=build_externals

Change-Id: I3c6c4280d06128b789e3a94c5c827c2630383e2a
Reviewed-on: https://chromium-review.googlesource.com/212139
Reviewed-by: Dan Shi <dshi@chromium.org>
Commit-Queue: Michael Liang <michaelliang@chromium.org>
Tested-by: Michael Liang <michaelliang@chromium.org>
1 file changed