Allow later dnspython releases

[fabaff]

At the moment dnspython is pinned. dnspython-2.0.0 was released a while ago.

Any change that newer dnspython releases are allowed?

[mschwager]

This is on my list of TODOs, although I'm not sure when I'll get around to it. I'd also like to do this to fix https://github.com/mschwager/fierce/security/dependabot/3.

Moving to the new async functionality in dnspython would improve performance as well. This is a better model for network-bound applications instead of multi-threaded.