Improve support for newer djangoes

Supported djangoes are now 2.2, 3.1, 3.2

Improved `changed_fields` API

• BACKWARD INCOMPATIBLE : The changed_fields kwarg passed to signals now contains field names as keys, not the field instances themselves.
• Dropped testing for old Django releases (now supported: 2.2.8+, 3.0.x)

Update python/django support

• Drop support for python 2
• Add support for django 3.0
• Add support for python 3.8

Django 2.1 support

• Added support for django 2.1
• Drop support for django 1.10 and earlier

0.3.1

Test on django 2.0

0.3.0

Drop support for django 1.9. Test on django 1.10 and 1.11

0.2.3

• Fix AppRegistryNotReady errors introduced in 0.2.2. Thanks @PetrDlouhy

0.2.0 - Support Django 1.8+

• Adds django 1.8 and 1.9 support
• Proper tests for all supported versions