Fix for: No module named builtins (CI tests error)

Author: xarx00

git/repo/base.py

@@ -4,7 +4,7 @@
 # This module is part of GitPython and is released under
 # the BSD License: http://www.opensource.org/licenses/bsd-license.php
 
-from builtins import str
+from __builtins__ import str
 from collections import namedtuple
 import logging
 import os