Simplify procedures for releasing and starting a version

[andy-maier]

Steps:

• Migrate to change fragment files using towncrier (PR #1481)
• Migrate to pyproject.toml with automatic version calculation using setuptools-scm. This requires introducing a start version tag M.N.Ua0. (PR #1486)
• Use the pushing of the start-version tag to trigger a new 'new_version' workflow that creates the PR for starting a new version and also merges it after the test workflow succeeds. Encapsulate the pushing of the start-version tag into VERSION=M.N.U make new_version.
• Encapsulate the releasing of a version into make release, using the final version calculated from the latest start-version tag in the checked-out commit history, and building the final change log file using towncrier.