We get this problem now: https://docs.readthedocs.io/en/stable/faq.html#i-get-import-errors-on-libraries-that-depend-on-c-modules . So we should resort to Mocking...
Solution attempt for #209 . Directly to master to trigger RTD (is otherwise very hard to test locally).
It may not work as there are also C-library deps. We may end up with a Conda dep file.