Option to select only notebook tests

[gasparka]

The pytest --nbval picks all the tests it finds. It would be useful to have an option to select only the notebooks. I found some ways to do this via .cfg file, but command line option would be better.
Ideas?

[vidartf]

You could skip python tests with the flags -p no:python, i.e. in full py.test -p no:python --nbval.

[vidartf]

Ref https://docs.pytest.org/en/latest/usage.html#disabling-plugins

[gasparka]

Thanks! Useful enough to add to the readme?

[takluyver]

I think it could be added to the README.