------------------
- Remove the AT schema extension and make it work within Plone 5.2
and Python 3. [Andreas Mantke]
- Increase the release number to 3 because it breaks compatibility
to old Archetypes content types. [Andreas Mantke]
- Added a test for value NOT_CHANGED to the validator module because
of a change in converter.py of plone.formwidget.namedfile [Andreas Mantke]
- isort and flake8 fixes in validator module [Andreas Mantke]
- Fix tests for Plone 5.2, discontinue Travis and switch to Github Actions
[tschorr]