Improve tests

[michaelarnauts]

Tests:

• Replace coverage with pytest-cov
• Remove tox
• Add tests for Library
• Run service with pylint instead of starting it directly

Github Actions:

• Don't run actions twice (push & pull_request), see https://github.community/t/how-to-trigger-an-action-on-push-or-pull-request-but-not-both/16662
• Use xbmc/action-kodi-addon-checker@v1.2 action instead of installing ourself
• Use output of xbmc/action-kodi-addon-submitter for the Github release