m2r is broken

[ZryletTC]

Docs aren't building because m2r is stale and fallen behind compatibility with the sphinx API.

Possible solutions:

• Could be fixed by m2r#55
• If desired, can be bypassed for now by pinning sphinx<3.0.0 or specifying the hotfix branch of m2r
• Realistically, that PR will never be merged as the repo has been inactive since 2018 so a better course of action would probably be to drop m2r and switch to recommonmark as others are doing

Originally posted by @ZryletTC in #400 (comment)

[ZLLentz]

I heard murmurs of sphinx making incompatible changes... My vote is always to either drop the dependency and do the docs a different way or to switch to a maintained package.

[klauer]

We use recommonmark in pytmc and have it suggested in the ci-helpers module. I don't recall it having ever gotten in the way.