Allow using repo with python3

* Switching from python2 to python3 in the same workspace isn't
  currently supported, due to a change in the pickle version (which
  isn't supported by python2)
* Basic functionality does work with python3, however not everything
  is expected to

Change-Id: I4256b5a9861562d0260b503f972c1569190182aa

repo

@@ -138,10 +138,9 @@ def _print(*objects, **kwargs):
 # Python version check
 ver = sys.version_info
 if ver[0] == 3:
-  _print('error: Python 3 support is not fully implemented in repo yet.\n'
+  _print('warning: Python 3 support is currently experimental. YMMV.\n'
          'Please use Python 2.6 - 2.7 instead.',
          file=sys.stderr)
-  sys.exit(1)
 if (ver[0], ver[1]) < MIN_PYTHON_VERSION:
   _print('error: Python version %s unsupported.\n'
          'Please use Python 2.6 - 2.7 instead.'