py-djangocms-style: updated to 2.3.0

2.3.0:
Added support for Django 3.0
Deprecated old CMS_STYLE_NAMES setting
Deprecated old CMS_STYLE_TAG_TYPES setting
Added further tests to raise coverage
Fixed smaller issues found during testing

2.2.0:
Added support for Django 2.2 and django CMS 3.7
Removed support for Django 2.0
Extended test matrix
Added isort and adapted imports
Adapted code base to align with other supported addons
Exclude tests folder from release build

(adam)