Unregisters a handler

def unhandle(self, handler):
        """ Unregisters a handler """
        h, _, _ = self._extract(handler)
        key = hash(h)
        with self._hlock:
            if key not in self.handlers:
                raise ValueError('Handler "%s" was not found' % str(h))
            handlers = self.handlers.copy()
            del handlers[key]
            self.handlers = handlers
        return self