Remove str import from builtins

Author: Harmon758

git/repo/base.py

@@ -4,7 +4,6 @@
 # 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 collections import namedtuple
 import logging
 import os