Revert of git_cl/gclient: use python2 (patchset #3 id:40001 of https://codereview.chromium.org/1437773002/ )

Reason for revert:
breaks git-bash on windows

Original issue's description:
> git_cl/gclient: use python2
> 
> Newer distros are defaulting /usr/bin/python to python3, so use python2
> explicitly so we continue working.
> 
> Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=297535

TBR=stip@chromium.org,sergeyberezin@chromium.org,vapier@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/1442583004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@297544 0039d316-1c4b-4281-b951-d872f2087c98