changed exact pins for minimum versions

[MarcAntoineSchmidtQC]

I don't know exactly why we pinned sphinx to a specific version. @tbenthompson maybe you remember?

I think this might solve the problem with the new readthedocs PR.

Checklist

• Added a CHANGELOG.rst entry

[MarcAntoineSchmidtQC]

see #512

[tbenthompson]

I think there was a temporary problem with that version of Sphinx that has likely been long since solved. But I don't remember precisely!

[jtilly]

Other projects have similar issues, see, e.g. microsoft/LightGBM#4957 (comment).

[astrojuanlu]

Alternatively, pinning python=3.9 in environment.yml might do the trick? Right now Python 3.10 is being picked up.