Drop support for Python < 3.7.

Changelog releaser now uses the unreleased name "TBD" instead of "Master".
CHANGELOG.md files must use "TBD" as the upcoming release name going
forward.

Support using branches named main as a default branch (previously default
branches had to be named master).

Repo commands now sort the list of repositories to provide a consistent
experience across macOS and Linux platforms.

Releasing no longer requires a git remote. Release can be used for local
repositories.

Release now offers a --verbose flag to log what is happening during
release.

GitHub releaser now allows you to upload artefacts to the GitHub release.

GitHub releaser will now upload changelog release from CHANGELOG.md files.

Hooks are now passed the VERSION as an environment variable.

Hooks can now be placed in files inside .maintain/hooks such as
.maintain/hooks/pre_release.

Changelog releaser allows you to specify custom sections.

release:
  changelog:
    sections:
      'Breaking': major
      'Enhancements': minor
      'Bug Fixes': patch

Configuration file can now be passed via the --config option.