Create CI tests workflow for the Python scripts in this repo

[anthonyfok]

Create quick CI tests for the Python scripts in this repo without having to do a full OpenDRR/opendrr-api stack build.

In the GitHub workflow that run these CI tests:

• Test multiple versions of Python: 3.8, 3.9 and 3.10 for starters
• Test on barebone GitHub runners with multiple OS versions: ubuntu-18.04, ubuntu-20.04, ubuntu-22.04
• Test also within mulitple versions of OpenDRR/python-env Docker images based on debian:sid-20201012-slim, debian:bullseye-slim (Debian 11 stable releases), and on debian:sid-slim.

Motivations: To allow, for example:

• Dependabot to test updated versions of Python libraries
• easier diagnostics for e.g. Issue columns expected but not found: ['structural', 'contents', 'nonstructural', 'asset_id'] #126

[anthonyfok]

See also https://realpython.com/python-testing/ which seems to be a really good introduction for me to introduce unittest etc. to model-factory Python scripts.