Fix #1838 tox testing on py38 and py39

[fnielsen]

This adds py38 and py39 options to the configuration file of tox.
It does not add them to automatic testing if tox is run without
environment parameter.

Fixes #1838

Description

We would like to be able to test with tox on the py38 and py39. This adds the options to tox.ini.

Caveats

Please list anything which has been left out of this PR or which should be considered before this PR is accepted
Check any of the following which apply:

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
  • I have made corresponding changes to the documentation
• This change requires new dependencies (please list)

If you make changes to the Python code

• My code passes the tox check, I can receive warnings about tests, documentation or both

Testing

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration.

• Test A
• Test B

Checklist

• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have not used code from external sources without attribution
• I have considered accessibility in my implementation
• There are no remaining debug statements (print, console.log, ...)

[Daniel-Mietchen]

Adding a pointer to

• Scholia hackathon on 14 and 16 February 2022 #1807

which serves to keep an overview of things we worked on during the hackathon.