Refactor changelog for the new release process

[ludoo]

This changes our changelog script to manage a single release, adding features for rc and multi-branch releases.

Copy/paste from the script docstring below.

This script allows adding or replacing individual release sections in our
CHANGELOG.md file.

It works on a set of simple principles

• a release is identified by two boundaries, the release for which we want to
  capture PRs (release_to or end release) and the release preceding it
  (release_from or start release)
• the end release can be null, to capture unreleased merged PRs (the
  'Unreleased' block in changelog)
• the start release can be null, to start capturing from the last published
  release (again to populate the 'Unreleased' block)
• merged PRs between the start and end times are captured and used to populate
  the release block
• PRs are grouped by their on: labels, and flagged as containing incompatible
  changes if they have one of the breaks: labels
• draft PRs or PRs merged against a base different from the one specified via
  the merged_to argument (defaulting to master) are ignored
• the unreleased block can optionally be marked with a release name via the
  release_as argument to prepare for a new release

Example usage:

• update the Unreleased section after PRs have been merged, both start and end
  releases use defaults, only PRs to master are tracked

  ./tools/changelog.py  --token=$TOKEN --write
  

• update an existing release on the master branch

  ./tools/changelog.py  --token=$TOKEN --write \
    --release-from=v34.0.0 --release-to=v35.0.0
  

• create a new release on the master branch

  ./tools/changelog.py  --token=$TOKEN --write \
    --release-from=v35.0.0 --release-as=v36.0.0
  

• create an rc release on the fast-dev branch capturing only branch-merged PRs

  ./tools/changelog.py  --token=$TOKEN --write \
    --release-from=v35.0.0 --release-as=v36.0.0-rc1 --merged-to=fast-dev
  

• create an rc release on the fast-dev branch also capturing master merged PRs
  e.g. when releasing an rc2 with fast-dev aligned on master

  ./tools/changelog.py  --token=$TOKEN --write \
    --release-from=v35.0.0 --release-as=v36.0.0-rc1 \
    --merged-to=fast-dev --merged-to=master
  

[sruffilli]

This process obsoletes part of the "cutting a new release" section of CONTRIBUTING.md, which should be updated at the same time.

I can take care of it after lunch.