Pywattbox

Add safety around missing ssh drivers.

Adjust optional dependencies to break out `ssh2-python`.

What's Changed
* Overhaul for async and ssh / telnet. by eseglem in https://github.com/eseglem/pywattbox/pull/3


**Full Changelog**: https://github.com/eseglem/pywattbox/compare/v0.4.0...v0.7.0