There should be a way to skip files during doctest collection.

I've given a more detailed description and motivation here:
#2310 (comment) .

I think DoctestPlus.pytest_ignore_collect (see here) currently overwrites the normal pytest collect_ignore mechanism (see here), because if I put

collect_ignore = ['docs/tutorials/image/sherpa_fit_image.py']

in the affiliated package gammapy/conftest.py it has no effect (or am I using it incorrectly?).

@embray Is it possible to restore the normal pytest collect_ignore functionality ... I think that's exactly what I need here.