avoid refcycles in tracebacks from happy eyeballs exceptions

[graingert]

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* Fix big bad boo-boo in task groups (#123 <https://github.com/agronholm/anyio/issues/123>_; PR by @yourgithubaccount)

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[graingert]

This is failing on Python3.9 because anyio is using asyncio.get_running_loop().create_connection() which in 3.9 didn't delete its reference to exceptions I think it should use sock.bind() + asyncio.get_running_loop().sock_connect instead.

[agronholm]

Just a couple nits.