make 'anyio' optional requirement.

[xiacunshun]

trafficstars

'anyio' is only used in tests, so there is no need to require it in main process.

Summary

Checklist

• [x] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [ ] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [ ] I've updated the documentation accordingly.

[waketzheng]

#3220