BUG: RTD uses old sphinx

[bsipocz]

v5.3.0 is I don't yet see why it does it. We use 3.10 for RTD, but even if I use that locally, tox manages picking up the latest release.

There is also a warning about the plot directive. I assume it is because we don't install matplotlib (as we don't have plots or plot directive in the docs), so that warnings should not be shown for us. I'll upstream that part to sphinx-astropy. xref: astropy/sphinx-astropy#70

/home/docs/checkouts/readthedocs.org/user_builds/pyvo/envs/latest/lib/python3.10/site-packages/sphinx_astropy/conf/v1.py:221: UserWarning: matplotlib's plot_directive could not be imported. Inline plots will not be included in the output