Test multiple Python versions through Docker build arg

[Darwinkel]

Changes

Adds a Docker build argument to change the base Python version, and adds a workflow matrix in the CI.

Issue ticket number and link

Linked.

Proof

See CI checks.

Checklist for author(s):

• This PR comes from a feature or hotfix branch, in line with our git branching strategy;
• This PR is "bite-sized" and only focuses on a single issue, problem, or feature;
• I am not reinventing the wheel: there is no high-quality library that already has this feature;
• I have changed the example .env files if I added, removed, or changed any config options, and I have informed others that they need to modify their .env files if required;
• I have performed a self-review of my own code;
• I have commented my code, particularly in hard-to-understand areas;
• I have made corresponding changes to the documentation, if necessary;
• I have written unit, integration, and end-to-end tests for the change that I made;

If a non-trivial PR:

• This PR is part of a milestone and has appropriate labels;
• This PR is properly linked to the project board (either directly or via an issue);
• I have added screenshots or some other proof that my code does what it is supposed to do;

## Checklist for functional reviewer(s):
- [ ] If a non-trivial PR: This PR is properly linked to an issue on the project board;
- [ ] I have checked out this branch, and successfully ran `make kat`;
- [ ] I have ran `make test-rf` and all end-to-end Robot Framework tests pass;
- [ ] I confirmed that the PR's advertised `feature` or `hotfix` works as intended;
- [ ] I confirmed that there are no unintended functional regressions in this branch;

### What works:
* _bullet point + screenshot (if useful) per tested functionality_

### What doesn't work:
* _bullet point + screenshot (if useful) per tested functionality_

### Bug or feature?:
* _bullet point + screenshot (if useful) if it is unclear whether something is a bug or an intended feature._

## Checklist for code reviewer(s):
- [ ] The code passes the CI tests and linters;
- [ ] The code does not bypass authentication or security mechanisms;
- [ ] The code does not introduce any dependency on a library that has not been properly vetted;
- [ ] The code does not violate Model-View-Template and our other architectural principles;
- [ ] The code contains docstrings, comments, and documentation where needed;
- [ ] The code prioritizes readability over performance where appropriate;
- [ ] The code conforms to our agreed coding standards.

[Darwinkel]

Ready for merge. A pydantic and fastapi upgrade is required for Python 3.11 support.

[Darwinkel]

Note: upgrading to fastapi >= 0.87 upgrades starlette as well, which replaces the requests library with httpx. httpx has different syntax and currently breaks some of our integration tests.