• Update InheritanceQuerySetMixin to avoid querying too much tables
• TimeStampedModel now automatically adds 'modified' field as an update_fields
  parameter even if it is forgotten while using save()
• FieldTracker now marks fields as not changed after refresh_from_db
• FieldTracker now respects update_fields changed in overridden save()
  method
• Replace ugettext_lazy with gettext_lazy to satisfy Django deprecation warning
• Add available_objects manager to SoftDeletableModel and add deprecation
  warning to objects manager.
• StatusModel now automatically adds 'status_changed' field during save as an
  update_fieldsparameter when 'status' is present in it to make sure it is not
  forgotten.
• Update test requirements
• Move tests to GitHub Actions: https://github.com/jazzband/django-model-utils/actions
• Drop support for Django 2.1
• Add support for Python 3.9
• Add support for Django 3.1