ES6 classes + minor maintenance tasks

[jukkah]

A list of unrelated maintenance tasks:

• Use ES6 classes ✨
• Update package data
• Make tests more stable
• Resolve deprecation warnings in tests

This contains small API changes (redefine classes => ES6 classes) but it should not be considered as a breaking change.

[mention-bot]

@jukkah, thanks for your PR! By analyzing the history of the files in this pull request, we identified @sdepold, @ruimarinho and @alubbe to be potential reviewers.