Fluent.migrate

- Use `git rev-parse` directly to check if the folder is within a git repository ([10](https://github.com/mozilla/fluent-migrate/pull/10)). This is needed to support migrations in a git monorepo instead of individual hg repositories.

- Add RepoClient as wrapper that auto-detects and supports both git & hg ([6](https://github.com/mozilla/fluent-migrate/pull/6))
- Add some type hinting
- Drop support for Python 3.7, add 3.11 and 3.12

- Trim plural variants in fluent.migrate `PLURALS()` ([Bug 1794286](https://bugzilla.mozilla.org/show_bug.cgi?1794286))
- Update dependencies to compare-locales 9.0.1 & fluent.syntax 0.19.0
- Add support for Python 3.8, 3.9 & 3.10
- Drop Python 2 support
- Drop Windows support
- Drop dependency on six
- Update contact details, including new [repo URL](https://github.com/mozilla/fluent-migrate)

- Change default behaviour for normalize_printf for .properties files

- Update to fluent.syntax 0.18.
- Update to compare-locales 0.8.1.

*Breaking Changes*

- [Bug 1616056](https://bugzilla.mozilla.org/show_bug.cgi?id=1616056) - trim by default for `LegacySource` transforms, but not inside `CONCAT`.

Other Changes:

- [Bug 1626976](https://bugzilla.mozilla.org/show_bug.cgi?id=1626976) - transforms can raise `fluent.migrate.errors.SkipTransform` to not transform a string, even if it exists in a localization.