Use the Agg backend for docs builds

[tdsmith]

This uses a non-interactive Agg matplotlib backend to build docs, which avoids trying to use the default MacOS backend, which can fail in some environments.

Closes #21913.

[codecov]

Codecov Report

Merging #21914 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #21914   +/-   ##
=======================================
  Coverage   91.96%   91.96%           
=======================================
  Files         166      166           
  Lines       50323    50323           
=======================================
  Hits        46281    46281           
  Misses       4042     4042

Flag	Coverage Δ
#multiple	90.36% <ø> (ø)	⬆️
#single	42.22% <ø> (-0.01%)	⬇️

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[jreback]

@jorisvandenbossche @TomAugspurger @datapythonista

[datapythonista]

Looks good, tested on Linux.

I was just guessing whether it'd be possible to set an environment variable in conf.py instead of creating a new file. Not a big deal, but there are already many files in doc/source and if it's the same, I'd prefer changing conf.py than adding a new file.

I'd say this link is about the same problem: https://stackoverflow.com/questions/25124977/cant-use-sphinxs-matplotlib-extension-with-qt-on-os-x#25164523

What do you think @tdsmith ?

[jorisvandenbossche]

Just to check, this has no influence on how the figures look in the docs? (the quality)

[datapythonista]

No quality difference that I can spot:

[jorisvandenbossche]

I would also assume this can be set in conf.py

[datapythonista]

Thanks @tdsmith, lgtm.

[tdsmith]

Thanks for the review, all! I just realized you asked for conf.py and not make.py -- it works for me in either place; AFAICT tweaking the environment is still the right thing to do in either place.

[jreback]

thanks @tdsmith