Un-subscribe to events matching the passed tag.

def unsubscribe(self, tag, match_type=None):
        '''
        Un-subscribe to events matching the passed tag.
        '''
        if tag is None:
            return
        match_func = self._get_match_func(match_type)

        self.pending_tags.remove([tag, match_func])

        old_events = self.pending_events
        self.pending_events = []
        for evt in old_events:
            if any(pmatch_func(evt['tag'], ptag) for ptag, pmatch_func in self.pending_tags):
                self.pending_events.append(evt)