Use xetex for uft8 latex backend

[Erotemic]

Closes #4325 This is a component of #4169

I finally settled on

latex_engine = "xelatex"
latex_use_xindy = False

Turns out xindy --- don't really know what that is --- was causing errors, and this setting allows xelatex to work (I think xindy is more a pdflatex thing?).

This PR does not have any dependencies

This PR is depended on by #4294, #4350, and #4327

[rossbar]

+1 for xelatex. Is that not enough to solve the unicode problems? I've never heard of "xindy" either.

[Erotemic]

I don't don't have deep knowledge about latex or sphinx and I know nothing of xindy, but I only got it to work after including texlive-xetex and texlive-fonts-extra in the apt get install and using those settings. Disabling xindy didn't have anything to do with unicode. I think xetex and more fonts is enough for that. I did also having it working with lualatex, but I arbitrarily chose xelatex instead.

[rossbar]

Looking into this a little more, it seems like the missing dependency for xindy is libffcall, so the options are either:

• disable xindy in conf.py, which may cause errors down the road if there are ever any index terms that contain unicode characters, or
• Add libffcall-dev to the CI scripts/configs responsible for building documentation.

It's a bit annoying that this requires another external dependency, but it's a small one and easy to install. I'd vote for the latter just because it reduces the chances of getting bit with LaTeX-unicode in the future.

Relevant sphinx docs on xindy: https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-latex_use_xindy

[Erotemic]

Based on the docs, it does look like we want to use xindy over the default makeindex when utf8 characters are enabled. Looks like you also need to sudo apt install xindy to make it work.

I tried installing libffcall-dev on my local machine and setting latex_use_xindy = True. This seemed to work. I'll push up a patch in a sec.

[rossbar]

A minor modification to the dependencies list and a suggestion for a less-verbose sphinx conf file. Other than those two things, I think this should go in for better unicode support (in the source) for our latex docs.

I tried this via docker and it worked, but we'll see if CircleCI agrees. If so, then I'll update the travis and gh-actions configs as well.

[rossbar]

Okay, looks like this worked (on CircleCI at least) so I updated the other CI configurations with the new latex dependencies. Thanks for the update @Erotemic !