Improve omission of notebooks from nbval test collection

[rossbar]

There are some notebooks that we don't necessarily want to test when running nbval, e.g. notebooks from the contributor guide, non-executable articles, etc. Currently this is handled manually in the CI job by simply deleting the notebooks we want to test:

numpy-tutorials/.github/workflows/notebooks.yml

Lines 36 to 40 in 47fb909

	# TODO: find better way to exclude notebooks from test
	rm content/tutorial-deep-reinforcement-learning-with-pong-from-pixels.ipynb
	rm content/pairing.ipynb
	rm content/tutorial-style-guide.ipynb
	rm content/tutorial-nlp-from-scratch.ipynb

This is inelegant and doesn't translate well to local workflows (i.e. users who want to run pytest-nbval on their own machines). It'd be a nice improvement to do this more cleanly - perhaps via a pytest configuration option to ignore files during collection?