Encapsulate releasing a version into single make command

[andy-maier]

Encapsulate the release of a version into a single make command, e.g. VERSION=M.N.U make release

That "make release" target should perform these steps:

• Check that the VERSION env var is set
• Check that the start version tag ${VERSION}a0 exists
• Determine the corresponding base branch (= head of the start version commit, i.e. master or stable_M.N)
• Check out the base branch (step 3)
• Create a topic branch for the release (step 3)
• Build the final change log using towncrier (step 4)
• Update the authors (step 5)
• Run the Safety tool (step 6)
• Commit and push the branch (step 7)
• Create a PR for the branch (step 8)

The existing "publish" workflow should get these changes:

• It is no longer triggered by pushing a release version tag, but by merging the release PR.
• It depends on the test workflow being complete
• It performs these additional steps:
  • Create release tag

The "Releasing a version" section in the docs would then become:

• Check that there are no open items in milestone ${VERSION} (step 1)
• Run VERSION=M.N.U make release
• Merge PR, once finished testing (step 9) -> this triggers the release to Pypi
• Close milestone ${VERSION} (step 10)
• Sync and cleanup the local repo (step 12)
• Verify the publishing (step 13)