Upgrade Python version

SASTADEV is developed against Python 3.7. This should be upgraded because:

- Python 3.7 has been end-of-life for a while now: https://devguide.python.org/versions/
- Forced server migration in the (very) near future, with new servers no longer suporting 3.7

I foresee no great consequences in the code itself, but the package dependencies may need thorough testing, with special attention for `lxml`.

Will comment soon with intended Python version. Prefer the highest supported by the new servers to ensure maximum support window.

No file, test, or entry point is named. First determine the Python version supported by the new servers, then inspect the project’s dependency and runtime configuration with particular attention to lxml; done means the project and its dependencies run successfully on the selected version.

Written by the indexing model from the issue text.