Python version, Dependabot and pipenv conflicts

[ml-evs]

Some fun, pipenv does not allow you to broadly specify a range of Python versions (e.g., ">3.8"), only "3" or something specific.
By default, dependabot uses a recent Python (3.10), which causes the lockfile to break for all versions that have different requirements between 3.8 and 3.10

Two possible solutions:

• Pin dependabot to the lowest supported Python version using .python_version or runtime.txt files (see Possibility to set Python version? dependabot/dependabot-core#1455 - still would be quite brittle).
• Only support one Python version for this project (would simplify quite a few things tbh, but could be restrictive on which dependencies we can use (e.g., if they don't support Python 3.10 themselves)).
• Scrap pipenv!

[ml-evs]

Closed by #604