Django-rest-form-fields

Features
1. Added ability to add underscore symbols in `UrlField`

Dependencies
1. Made `pytz` and `jsonschema` optional dependencies

CI
1. Replaced python 3.6 with python 3.11, 3.12

Bug fixes
1. Fix ModelForm issue in django 3.0+, which caused fields with required=False validation by model fields

Testing
1. Replaced django 2.1 with django 4.0 in test matrix

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

Features
1. Dependencies optimization