fix: Unpin mistune, docutils, and sphinxcontrib-openapi

[timmc-edx]

This allows dropping the unmaintained m2r package.

• Was blocked on m2r haven't been updated in years sphinx-contrib/openapi#123
• May be worth backporting to Olive (...if tests pass)

[timmc-edx]

@UsamaSadiq I saw that the relevant openai issue had been closed, so I tried unpinning that and a few related dependencies to see what would happen. If the docs build passed, does that mean we're good to go? Or was the error you observed here sphinx-contrib/openapi#129 (comment) seen in some other context?

[UsamaSadiq]

Hi @timmc-edx, arbi-bom has created PR (#31574) to make this change already. That PR will be merged & deployed today so this PR won't be needed any more.
Please let us know if we'd need to backport this change to Olive as well.

[timmc-edx]

Ah, wonderful! Closing this one out.

And yes, I think backporting to Olive would be a good idea -- the fewer constraints we have in general, the easier it is to get security upgrades to dependencies.