Use latest sphinx version for the `doc` build

We are using the minimum version of sphinx when building the documentation:

https://github.com/scikit-learn/scikit-learn/blob/86bda0ae8472687e19bc071c4cbb957a21738650/.circleci/config.yml#L58

This probably a good thing when the CI temporarily fails and it gives us time to adapt our build script. However, now we are pinning sphinx to 3.2.0 while sphinx is about to release 4.0. Thus, we are lagging quite behind without any clue regarding the build issue.

I would advocate using 'latest' for the doc build whenever possible and if we need to pin the version to add a comment with the reason with an attached issue such that we can keep track of the problem.

[thomasjpfan]

We recently added a dependency on sphinxext-opengraph. You can install it with pip install sphinxext-opengraph. (You should be able to use the latest version of sphinx)

REF: https://scikit-learn.org/dev/developers/contributing.html#building-the-documentation

[MichaelShoemaker]

I realize this is closed, but is there a work around for this issue? I was hoping to be able to help with some of the documentation issues, but I recieve the error:

sphinx.errors.ExtensionError: Could not import extension sphinxext.opengraph (exception: No module named ‘sphinxext’)

After uninstalling sphinx I tried pip install sphinx==1.8.3, but receive the same error. Any guidance is greatly appreciated.