> This 'check phase doesn’t respect the ‘--with-tests’ transformation.
> Something like the following would be better:
>
>   (replace 'check
>     (lambda* (#:key inputs outputs tests? #:allow-other-keys)
>       (when tests?
>         (with-directory-excursion "tests"
>           (add-installed-pythonpath inputs outputs)
>           (invoke "py.test" "tests")))))

Makes sense. I'll add the `when tests?', but what's the
with-directory-excursion for?