Django-rest-form-fields

Tests
1. Moved testing from Travis to GitHub Actions
2. Linter fixes

Features
1. Dependencies optimization

Bug fixes
1. Fixed issue https://github.com/M1hacka/django-rest-form-fields/issues/14

Testing
1. Added python 3.8

Features
Added DateField

Bug fixes
1) Removed python 3.3 support as it is not supported by psycopg2 any more
2) Fixed python 3.7 compatibility

Bug fixes
1) Fixed bug in RestChoiceField which coused exception when source was set

Bug fixes
1) Fixed bug in FileField exceptions, so they are valid ValidationError instances ([issue 7](https://github.com/M1hacka/django-rest-form-fields/issues/7))