readthedocs build failing

[jcristau]

readthedocs was still building "master" instead of "main". After changing that the "latest" build fails with:

Error

Problem in your project's configuration. Invalid "python.version": expected one of (2, 2.7, 3, 3.5, 3.6, 3.7, 3.8, pypy3.5), got 3.1