Add support for Django 3.1

[martinfrancois]

"Adding support" may be a very generous term here, I didn't need to make any changes to the codebase, it was already compatible and all of the tests are passing :)

Description

• Run unit tests on Travis CI with Python 3.6, 3.7 and 3.8 and Django 3.1
• Add Django 3.1 as supported in README and setup.py

Related Issue

Not sure this warrants discussing in an issue - if this is the case, I don't mind opening one up, just tell me.

Motivation and Context

It allows all users of django-simple-history that want to upgrade to Django 3.1 to upgrade with the confidence that it's supported and helps prevent regression bugs that may be introduced in the future that only affect Django 3.1

How Has This Been Tested?

I ran the tests locally and on Travis CI.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have run the make format command to format my code
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have added my name and/or github handle to AUTHORS.rst
• I have added my change to CHANGES.rst
• All new and existing tests passed.

[rossmechanic]

Thanks for this @martinfrancois

[martinfrancois]

You're welcome @rossmechanic, thanks for the quick response!