Add Python 3.12 testing to continuous integration workflow

[rowanhemsi]

Is your feature request related to a problem? Please describe. I want to feel confident that gptables will work with Python 3.12, which is the latest stable version of Python as of October 2023.

Describe the solution you'd like CI workflows check build and tests on Python 3.12

[gisellerosetta]

We might want to review these in general, looking at the status of python versions.

[gisellerosetta]

Changed title to reflect broader changes in workflows.

Definition of Done:

• Workflows no longer include Python 3.8 and below.
• Workflows use consistent Python versions, e.g. tests on the different operating systems run for the same Python versions.
• Workflows test Python versions within the range of 3.9 - 3.13 (may not need all of them if this uses excessive CI minutes).
• Test coverage returns coverage for the most recent Python version.