Test multiple Sphinx and OS versions

[hugovk]

Fixes #36.

Includes PR #67 to avoid conflicts.

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#36 (comment) says:

We should test on all platforms.

The workflow should be:

1. ubuntu python3.6 builds the wheel and uploads it as an artifact
2. [win, mac, ubuntu] x [py 3.6, 3.7, 3.8, 3.9] download and test the wheel
3. ubuntu uploads the wheel

So let's do this, except using Python 3.7 instead of removed EOL 3.6, and testing the wheel on 3.7-3.11.

• Was there a particular reason 3.6 was suggested for building the wheel? Should we use a newer one like 3.10?

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Further, #36 (comment) says:

We should also test on major supported sphinx versions

So let's also test on Sphinx 4, 5 and 6 beta.

• I did try Sphinx 2 and 3 as well but they failed due to incompatibilities of Sphinx dependencies, and it's probably not worth supporting such old versions. It would make the matrix even bigger than the current 51(!) jobs and be more complex as they don't support 3.10+ either.

• The matrix could be reduced by only testing oldest and newest Python stable versions, plus RC + PyPy (i.e. 3.7, 3.10, 3.11-dev, pypy3.8)

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https://github.com/hugovk/sphinxext-opengraph/actions/runs/3263561876

[hugovk]

These required checks will need updating:

Because it can be tedious work removing and adding these in the settings when Python versions change, one option is to have a final no-op job that depends on the others. And then just mark that final as required.

For example:

https://github.com/hugovk/norwegianblue/blob/8cb61db6dd8a7f5d4e19c389eb91a70c820354d7/.github/workflows/test.yml#L49-L55

https://github.com/hugovk/norwegianblue/actions/runs/3177642324

Would you like something similar here?

[Daltz333]

Merging, and an issue can be opened for continue discussion. Gotta update the CI checks. Thanks!