Create changelog automation

[SchoolGuy]

To ensure the quality of releases we want to use Towncrier for changelogs.

Links:

• https://towncrier.readthedocs.io/en/stable/
• https://github.com/twisted/towncrier

It should use the same automation as koan in this regard.

Short summary:

• One file per release.
• Automatic creation of a PR for building the changelog.
• Automatic check if a PR has a changelog entry or not (see here).