chore: versioning and changelog generation setup

[mih]

This is a commitizen-based setup.

Two commands are provided for hatch users (and documented for others) in pyproject.toml:

• cz:show-changelog
• cz:bump-version

Both are (also) concerned with changelog generation. The changelog style is, in principle, similar to other setups we have. However, no pointers to PRs are included.

This is done, because adding critical information to any PR that is only available at a specific service, and is not part of the code base, is strongly discourages. All relevant information should be in the sources, all relevant meta-information should be in commit messages.

Consequently, changelog pointers are pointers to commits.

For convenience, GitHub commit page links are included nevertheless. Such links are be easily replaced in bulk when migrating services.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (cef3ed3) to head (b21c8ea). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #7 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 2 2 Lines 4 4 ========================================= Hits 4 4 ```

:umbrella: View full report in Codecov by Sentry.
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