Unregisters the given class.

        If the specified class isn't registered, ``NotRegistered`` will
        be raised.

def _unregister(self, cls):
        """Unregisters the given class.

        If the specified class isn't registered, ``NotRegistered`` will
        be raised.
        """
        if not issubclass(cls, self._registerable_class):
            raise ValueError('Only %s classes or subclasses may be '
                             'unregistered.' % self._registerable_class)

        if cls not in self._registry:
            raise NotRegistered('%s is not registered' % cls)

        del self._registry[cls]

        return True