The readthedocs service we use appears to be broken at the moment. Issue reported here (and in a dozen or more other places):

There is a workaround, to pin the docutils<0.18 version, but I think better to just wait for readthedocs to fix the issue as it is affecting so many builds.

If they haven’t fixed it in the next few days then I’ll try to apply the workaround.

Regards, Mark.