PR: Pin Python version on CIs to 3.9 to fix build failures with 3.10

[CAM-Gerlach]

Pull Request

Pull Request Checklist

• [x] Read and followed this repo's Contributing Guidelines
• [x] Based your PR on the latest version of the correct branch (master or 4.x)
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• [x] Described your changes and the motivation for them below
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Description of Changes

Currently, the CI builds are now failing because the Python 3.x target shifted to 3.10 in the latest runners. Unfortunately, we're pinned to Sphinx 3.x due to Sphinx-Multiverison, which doesn't support Python 3.10. Accordingly, I've pinned the runners to use Python 3.9, which should fix the builds and at least give us a few years to figure out what we want to do.

Discovered in and currently blocking #301

[CAM-Gerlach]

Merging since this is a trivial but critical fix needed to get the CIs working, and blocking work on #301 .