Ready for SQLAlchemy 0.9: changes in Events._remove() interface

[ods]

Interface of some private methods changes:

  File "testalchemy.py", line 210, in __exit__
    event.Events._remove(self._target, 'after_flush', self._after_flush)
TypeError: _remove() takes exactly 2 arguments (4 given)

[ods]

Related issue in SQLAlchemy: http://www.sqlalchemy.org/trac/ticket/2832 (Update: already fixed).