Work around a 2to3 bug more simply. PYTHON-494

Author: A. Jesse Jiryu Davis

bson/__init__.py

@@ -525,7 +525,7 @@ def decode_all(data, as_class=dict,
  raise e.with_traceback(exc_tb)
  else:
  # 2to3 mistranslates this.
- raise InvalidBSON, InvalidBSON(str(exc_value)), exc_tb
+ raise InvalidBSON, str(exc_value), exc_tb
 
 if _use_c:
  decode_all = _cbson.decode_all