readthedocs: pdf render fails

[ddkohler]

Example here:

https://readthedocs.org/projects/wrighttools/builds/23532744/

Last working example here:
https://readthedocs.org/projects/wrighttools/builds/23490585/

Some of the issues that come up:

• bad links to images
• graphics extension svg not currently supported (need an additional library?)
• unicode characters from code-blocks introduced in reworking wright-tools entry points #1152 are not supported

We will turn of pdf renders in the meantime.