fixed python 2.6 compatibility.
fixed python 2.6 compatibility.

flask-admin/flask-admin,jmagnusson/flask-admin,flask-admin/flask-admin,flask-admin/flask-admin,rochacbruno/flask-admin,rochacbruno/flask-admin,rochacbruno/flask-admin,rochacbruno/flask-admin,jmagnusson/flask-admin,jmagnusson/flask-admin,flask-admin/flask-admin,jmagnusson/flask-admin