Remove checks for Python 3.8, document that 3.8 or higher is needed and test on >3.8

[simonprickett]

The Makefile currently requires Python 3.8. Remove this hard requirement (probably also the .python-version file) and make sure that the environment builds and tests on Python >=3.8 (e.g. try with 3.9/10/11). Will require update to the README file to say what the minimum version of Python required is.

Removing tests for Python version in favour of documenting the minimum required Python version seems to be a better way to go now.