readthedocs documentation is outdated

[trixmoe]

The commits for readthedocs are 89ecd35 for latest or d0a9dff for stable and master.
Both commits are from 2016.

There is also a missing .readthedocs.yaml in the repository, which seems to be required nowadays.

I'm trying to make some changes to the docs, but it is difficult to re-build it, nor make it work in a testing readthedocs.

• Upstream readthedocs does not have Python 3.5 anymore
• It fails building when using the pre-existing requirements file due to psutil (Building wheel for psutil requires clang).
• With clang given or psutil removed, building fails when requests checks compatibility w/ urllib3, chardet_version, charset_normalizer_version.