fix(test): deal with py2 and py3

It ain't pretty, but should do the job.

Related to gitpython-developers#514

Author: Byron

git/test/test_index.py

@@ -145,7 +145,9 @@ def add_bad_blob():
             ## 1st fail on purpose adding into index.
             add_bad_blob()
         except Exception as ex:
-            assert "cannot convert argument to integer" in str(ex)
+            msg_py3 = "required argument is not an integer"
+            msg_py2 = "cannot convert argument to integer"
+            assert msg_py2 in str(ex) or msg_py3 in str(ex)
 
         ## 2nd time should not fail due to stray lock file
         try: