Reviewed support for pytest

Do not merge this one: merge @ftobia's one (after it includes these changes).
I created this one because I couldn't figure out how to pull-request changes to ftobia's branch...
Perhaps I could have done it via my personal account (benoitbryon/hospital instead of reference python-hospital/hospital).

Whatever... @ftobia, does this review sounds good to you? I may merge it on monday. Feel free to update your own pull-request with these changes (merge "origin 50-support-pytest-reviewed" into your branch). Then add some changes of your own ;)