Fixed compatibility of __str__ with py3
Fixed compatibility of __str__ with py3

rmoorman/sqlalchemy-utils,tonyseek/sqlalchemy-utils,JackWink/sqlalchemy-utils,tonyseek/sqlalchemy-utils,cheungpat/sqlalchemy-utils,marrybird/sqlalchemy-utils,konstantinoskostis/sqlalchemy-utils,joshfriend/sqlalchemy-utils,joshfriend/sqlalchemy-utils,spoqa/sqlalchemy-utils