Sphinx version update

[AngryMaciek]

Great Action! Could you please put out a new minor version with Sphinx 3.x?

[ammaraskar]

Hey Maciek, I'd like to solve this in a way that isn't a breaking change. I believe Sphinx 3.x has a lot of potential incompatibilities with 2.x and a lot of people are using the version pinned to master, i.e ammaraskar/sphinx-action@master

Would a change that lets you configure the sphinx version in the settings be good as well? I'm thinking something like:

      with:
        sphinx-version: "3.5.4"

[AngryMaciek]

Sure! :)

[StijnCaerts]

Support for Sphinx 3 or even the newly released v4 would be very much appreciated!
I don't know if it would be possible to configure the version of the Docker image used in a parameter, see this Stackoverflow thread: https://stackoverflow.com/a/58478925/9339603

But it should be possible to support this on a separate branch, without affecting users relying on ammaraskar/sphinx-action@master: https://docs.github.com/en/actions/creating-actions/about-actions#using-branches-for-release-management

[cosunae]

just do add to the discussion, while using this action we recently triggered a bug
sphinx-doc/sphinx#9727
where the recommended solution is to upgrade the sphinx version, recent versions v4.0 provide a bugfix.
So support for recent versions of sphinx would be very much appreciated

[astrojuanlu]

Having a

      with:
        sphinx-version: "3.5.4"

setting would be perfect.

[ptth222]

I was able to add "sphinx>=5.1.1" to the requirements.txt file and get pip to upgrade sphinx, so this is a decent work around for now I think.