Durable authentication implementation + Multiple fixes to current functionalities

[etmelvin]

• Implemented durable authentication described in this issue: Need to provide durable authentication galak/span-hacs#11
  • Config flow now directs user to open/close their Span Panel to get temporary authorization for access token request
  • Subsequent requests to the Span Panel uses that access token
• We now check for invalid responses from Span Panel before passing that over to Home Assistant
  • Your energy dashboard should no longer record 0 when that happens, instead it’ll just ignore that update loop
• Fix an issue with the button toggle and priority selector not working properly
  • There is a API change where the payload key was changed from snake_case to lowerCamelCase
  • I couldn’t validate that priority selector is working properly after the fix as I’m getting strange 500 errors, this might be because I don’t have batteries for my system; I can’t change those values from their app either
• Did some refactoring too based on python standard black + isort

[gdgib]

Tested working for me, with the exception of the same 500 errors. I agree with the guess that those are related to not having a battery, as I can confirm the very same 500 error is returned to the SPAN built-in web-app. In any case, that sure proves this isn't your/our fault.

This is nice work @etmelvin ! Thank you very much.