Docs are failing to build

[makermelissa]

Here's the message:

Running Sphinx v1.8.5
loading translations [en]... done
making output directory...
loading pickled environment... done
building [mo]: targets for 0 po files that are out of date
building [latex]: all documents
updating environment: 0 added, 0 changed, 0 removed
looking for now-outdated files... none found
processing AdafruitPlatformDetectLibrary.tex...
Exception occurred:
  File "/home/docs/checkouts/readthedocs.org/user_builds/adafruit-platformdetect/envs/stable/lib/python3.7/site-packages/sphinx/builders/latex/__init__.py", line 201, in write
    if toctrees[0].get('maxdepth') > 0:
TypeError: 'generator' object is not subscriptable
The full traceback has been saved in /tmp/sphinx-err-kkelhqqh.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!

I'm not sure if it's something on our end or readthedocs' end.

[makermelissa]

Ok, looking at other recent repos, this appears to be happening on a scale that affects more than this repo.

[makermelissa]

Ok, I found this: readthedocs/readthedocs.org#8616 (comment)
Looks like the solution is to pin docutils<0.18. This is done with a separate requirements.txt for the docs and a .readthedocs.yaml file. It looks like in older repos, we are using the older format of .readthedocs.yml, but it is missing from PlatformDetect.