Upgrade to Python 3.10; Upgrade Sphinx to 4.2.0

[jertel]

Description

ElastAlert 2 is pinned to Python 3, so whenever a new minor release of Python 3 is uploaded to Docker Hub, the next time ElastAlert 2 builds it will use the newer minor release. This is good in that it keeps ElastAlert 2 current, but when an incompatibility arises, the builds and tests fail. This occurred most recently with PR #499. That PR will not have a clean build until ElastAlert 2 is either back pinned to Python 3.9, or is upgraded to fully support Python 3.10.

This PR adds supports for Python 3.10, which required installing gcc and libffi-dev packages in order to build the cffi python wheel. Also, Sphinx 4.1.2 has an incompatibility with Python 3.10, which is fixed in Sphinx 4.2.0.

Checklist

• I have reviewed the contributing guidelines.
• [N/A] I have included unit tests for my changes or additions.
• I have successfully run make test-docker with my changes.
• I have manually tested all relevant modes of the change in this PR.
• [N/A] I have updated the documentation.
• I have updated the changelog.

Questions or Comments

[nsano-rururu]

Although it is sphinx_rtd_theme, since it supports docutils 0.17, I think that there is no problem by deleting the description of docutils<0.17 from requirements-dev.txt.

Add support for docutils 0.17
readthedocs/sphinx_rtd_theme#1185