A minor fix after porting git-python over to PY3

It doesn't do anything (in terms of fixing an issue), but it should be more correct than what was there previously

Author: Byron

gitdb/utils/encoding.py

@@ -22,7 +22,7 @@ def force_text(data, encoding="utf-8"):
     if isinstance(data, text_type):
         return data
 
-    if isinstance(data, string_types):
+    if isinstance(data, bytes):
         return data.decode(encoding)
 
     if compat.PY3: