🐛 FIX: Path to custom output directory for _sources

[dfm]

In #339 I introduced a fix for the _sources directory path that works with the default sphinx Makefile. But I have since discovered that this doesn't work with a custom output directory (as used by ReadTheDocs, for example). This updated PR should do the trick more generally. Sorry about the noise!

[welcome]

Thanks for submitting your first pull request! You are awesome! 🤗

If you haven't done so already, check out EBP's Code of Conduct and our Contributing Guide, as this will greatly help the review process.

Welcome to the EBP community! 🎉

[choldgraf]

oops I missed this PR, sorry! Is there a way that we can confirm this really solves the problem this time? E.g., is there a test that could make it fail w/o this PR, and passes with this PR?

[dfm]

No worries at all! I'll do some poking around and see what I can come up with.

[dfm]

@choldgraf: ok - I added a test that checks to make sure that the ipynb generated for a myst notebook ends up in the right _sources directory. It uses subprocess because I couldn't figure out how to pass outdir to sphinx_build_factory. Happy to update if there is a better way. This fails on master but passes with this fix. Thanks!

[choldgraf]

Nice - that seems reasonable, I added in a comment to make it clear why we're using sphinx-build there. Thanks for this improvement!