Add unit tests to validate the cookiecutter template

[lucasmelin]

The python project template needs to work flawlessly out of the box, so we should make sure that we're regularly testing that the cookiecutter still works.

We should check that we can:

• [x] Generate a regular python project
• [x] Generate a FastAPI project
• [x] Run the included unit tests, linters and formatters without errors
• [x] Generate documentation with mkdocs

We should also consider checking for out-of-date dependencies.

These checks should run on PR, and also periodically, maybe once a week.

[lucasmelin]

As a starting point, we can reference https://til.simonwillison.net/cookiecutter/pytest-for-cookiecutter for setting up some tests.