Fix PDF build on RTD

[stevepiercy]

I broke the PDF build on RTD on the master branch. It looks like they don't support xelatex yet, but they're close.

I might have to revert bits of #3288.

I like having nice tree diagrams in the HTML, but it's at the expense of a successful PDF build on RTD. The PDF builds locally.

[mmerickel]

@stevepiercy you need to pin to an older sphinx if necessary or whatever else it takes (I'm not super clear what's going on) to get the build working again... We can't leave our docs badge set at "failing" and every html build of rtd saying failed. https://readthedocs.org/projects/pyramid/builds/

[stevepiercy]

It requires reverting the tree diagrams to ASCII characters. It looks like the open issue on RTD has stalled. Bummer.

[stevepiercy]

Until RTD resolves Xelatex for PDF generation, I'll turn off PDF generation for Pyramid in the RTD Advanced settings. Users can always build a PDF locally. If there is a demand for it, then someone can submit a PR to revert the tree diagrams.

I've also removed this issue from the 1.10 milestone.

[stevepiercy]

Some progress has been made. I am now tracking readthedocs/readthedocs.org#5453

[stevepiercy]

It has been fixed per readthedocs/readthedocs.org#5503. I will backport to 1.10-branch and 1.9-branch from master, and add a changelog entry.

[stevepiercy]

Closed by #3472, #3473, #3474, and #3475.