List supported Python versions

Description

The Python version(s) supported should be provided in the project, either in the documentation or as a "badge".

I did find in the Developer Manual under Installation that Python 3.6 was being used but this wasn't stated explicitly in the project.

Requesting this since I was working to add a Dockerfile and found quickly that using the python:3 image gave me Python 3.9, which is not yet supported (due to numpy requirement not being met). However, Python 3.8 did work.

This issue is essential as part of my review for JOSS.