execute doctests in our continuous integration setup

[ctb]

some results from #889 make me wonder - more soon.

[luizirber]

Oops, apparently not. Seems to be missing --doctest-modules in pytest.ini:
https://github.com/dib-lab/sourmash/blob/a1eeab17ed424c35d58f0d1dc74420eb246d70a4/pytest.ini#L2

[ctb]

I believe this now involves modifying the [tool:pytest] section of setup.cfg to have --doctest-modules.