Rollback Sphinx version to 2.4.4

[oatawa1]

What this patch does to fix the issue.

There is no fix version of Sphinx, thus it always keeps updated to the latest version, or keeps using the currently installed version in the environment.

Now Sphinx==3.0.0 is just released and contains some bug which leads to document generation errors.

So I fix the Sphinx version to the latest useable 2.4.4.

Link to any relevant issues or pull requests.

Closes #968

Other people got this issue as well.
sphinx-doc/sphinx#7429

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[iizukak]

OA

[iizukak]

/ready

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[oatawa1]

@iizukak Thank you, the document is working now. 🎉