Fix no_proxy to support IPV6 CIDR format

[p3ck]

Summary

Fixes no_proxy to support IPV6 CIDR format.

no_proxy=fe11::/16 python -c 'import httpx; c = httpx.Client()'

Closes: #3221

Checklist

• 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.