CI: fix RTD builds on push to master

[consideRatio]

It appears that when we build the docs in the PR preview, it works. But,
when we push to master, it doesn't any more. It appears that using
pydata_sphinx_theme to build PDFs and/or ePub files makes it crash if we
using sphinx3 rather than sphinx2. Since a doc dependency that
(executablebooks/myst-parser) previously was limited to sphinx 2 but not
longer is, the PDF / ePub build made our builds when pushing to master
fail.

I'd like to avoid maintaining these things, and would like to stop
building PDFs and ePub files. I don't know if they are used, but I'd
like to focus attention to https://z2jh.jupyter.org anyhow I think.

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I'll self merge this now as to trial that this actually resolved the issue. If we want to have htmlzip / PDF / ePub builds again, I think we can pin sphinx<3 in doc-requirements.txt and configure formats: all in .readthedocs.yml instead of formats: [].

Here is an related issue about the build failure, which stems from the interaction of pydata_sphinx_theme and RTD builds like htmlzip it seems: pydata/pydata-sphinx-theme#177

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Update

The build works!

[consideRatio]

@jupyterhub/jupyterhubteam, do we wish to keep putting in work to ensure htmlzip, pdf, and ePub builds to function with our documentation, or only do standard html builds?

[manics]

I didn't know we had zip/pdf/epub builds, nor can I think of a situation where I'd want to download those formats.