Mail-deduplicate

- Reconcile `v3` branch with `develop`.

- Simple re-release.

- Add retroactive support for Python 3.6. Closes {issue}`154`.
- Fix documentation link and generation. Closes {issue}`66`.
- Auto-generate API documentation via a GitHub action workflow.
- Add `tomlkit` dependency.
- Add test runs against new OSes and distributions: `ubuntu-18.04` and
`macos-11.0`.
- Remove `pycodestyle`, it brings nothing more now that we rely on
`black`.

- Load up all subfolders from `Maildir` and `MH` boxes. Closes {issue}`123`.

- Check early that `--export` file doesn't exists. Closes {issue}`119`.
- Add screenshots.

- Add new `-a`/`--action` option to choose what to do on the final mail
selection.
- Implements new `copy-discarded`, `copy-selected`, `delete-discarded`,
`delete-selected`, `move-discarded` and `move-selected` actions.
- Add new `-E`/`--export` and `-e`/`--export-format` options to support
the new `copy-*` and `moved-*` actions.
- Rename all `--delete-*` strategies to `--discard-*`.
- Add `--select-*` aliases to all strategies.
- Add new `discard-all-but-one`, `discard-one`, `select-one` and
`select-all-but-one` selection strategies.
- Rename `-f`/`--sources-format` option to `-i`/`--input-format`.
- `--time-source` parameter is now optional and defaults to
`date-header`.
- Add metric description in deduplication end report.
- Add detailed strategy description in help screen's epilog.
- Colorize help screen.
- Colorize version screen and print environment data for bug reports.
- Run tests on Python 3.9.