Newer distros are defaulting /usr/bin/python to python3, so use python2
explicitly so we continue working.

Review URL: https://codereview.chromium.org/1437773002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@297535 0039d316-1c4b-4281-b951-d872f2087c98