[issue23209] asyncio: break some cycles

[STINNER Victor]

STINNER Victor added the comment:

I opened the issue #23225 "selectors: raise an exception if the selector is closed" which is a different approach (but it should also fix the reference cycle, I kept the "self._map = None" change).

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue23209>
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