Django-heroku-connect

- migrate to GitHub actions, add python 3.9 and Django 3.1 (104)
- [doc] Add a note on Django signals (105)
- Replace deprecated Django `ugettext_lazy` (106)

- change field-type for values on trigger-log tables ( 103 ) **backwards incompatible change!**
- new triggerlog `.redo` functionality broke when retrying on objects with read-only fields ( 102 )

Replace `models.NullBooleanField` usage in favor of `models.BooleanField(null=True)` (Django 4.0 deprecation warning)

- use recapture when doing `redo` on ordered-write errors 98

changes:
- revert the index on `is_deleted` fields (https://github.com/Thermondo/django-heroku-connect/pull/97)
- make wrong db-contraints on FK an error, not a warning (https://github.com/Thermondo/django-heroku-connect/pull/94)

Added

- `MANIFEST.in` for a better package

Changed

- Drop Travis matrix to unblock Travis stages (this was also preventing the PyPI release process)