MNT: Pin sphinx to avoid very slow doc build on RTD

[pllim]

Description

This pull request is to see what is causing RTD doc build to take so long.

Also see readthedocs/readthedocs.org#7648 #10807 #10998

Note: Cancelled CI on purpose as I don't think they are needed.

[pllim]

@saimn , I don't see the timing. Is simply adding the extension not enough? Is there something else I need to do? The documentation is pretty bare. https://www.sphinx-doc.org/en/master/usage/extensions/duration.html

[saimn]

Yes, you just need to add the extension, but it doesn't seem to work on RTD (I noticed the same thing in #10795).

[saimn]

Also, having a look at the builds on RTD, I don't see what could have changed on Astropy side to explain the the build takes more than twice longer. 2 weeks ago sphinx-build was taking 500-600 seconds.

[pllim]

I have my suspects...

	Older log (faster)	New log (slower)	This PR log
Build ID	12228280	12305403	12306918
Build date	2020-10-30	2020-11-10	2020-11-10
sphinx-build duration (s)	536	1345	562
Sphinx	3.2.1	3.3.0	3.2.1
Matplotlib font message	--	"Matplotlib is building the font cache; this may take a moment."	"Matplotlib is building the font cache; this may take a moment."
cached-property	1.5.2	--	--
attrs	20.2.0	20.3.0	20.3.0
kiwisolver	1.2.0	1.3.1	1.3.1
numpy	1.19.3	1.19.4	1.19.4
scipy	1.5.3	1.5.4	1.5.4
sortedcontainers	2.2.2	2.3.0	2.3.0
toml	0.10.1	0.10.2	0.10.2

[pllim]

I think the culprit is Sphinx 3.3.0 (released 2020-11-02) but I don't know if this is Sphinx bug or our sphinx-astropy isn't playing well with new Sphinx release.

[pllim]

I wonder if I should open an issue over at Sphinx but I wouldn't know how to isolate the problem for them.

Update: Reported to sphinx-doc/sphinx#8403 as Brigitta suggested.

[bsipocz]

Hmm, even just reporting them this table and the logs might be useful enough for them.

(I also wonder whether the mpl font building also has any effects, I recall it was an issue in the past?)

[pllim]

@bsipocz , font building seems to add about 30-ish seconds but definitely was not the cause.

[saimn]

Sounds good, thanks @pllim for the investigation !
I guess we could merge this soon ?

[pllim]

I guess we could merge this soon ?

Yes, please!