v3.0.0: latest stable release

This release includes lots of changes: 71 commits, 92 lines of commit messages and 3 reviewers (thanks to @Griatch, @Menda and @esquonk). This release is backward incompatible and includes refactoring, improving code readability (docstrings, annotations, ...), tests, new shiny features and the like. It started as a task to refactor the code, but ended with many other things.

The biggest change so far was adding tests along with GitHub Actions as CI and the introduction of SP-initiated SSO. The exception and error handling is also greatly improved. For a more detailed description of what happened in this release, please read the changelog.