Lastcast

- Drop support for Python versions before 3.7
- Fix support for Python 3.9 via dependency upgrade (thanks dewdropawoo!)
- Bump dependencies to latest

- Drop support for Python 3.5 and earlier
- Bump PyChromecast version to fix support for discovery inside Docker container (fixes 53 - thanks jbmfg!)

- Don't scrobble advertisements for free-tier Spotify users.

- Add Youtube Music as a default source to scrobble from (thanks mone!)

- Hide debugging output by default (and add `--verbose` to show it)
- Minor refactoring + style nits