Test against Python 3.9

[duckontheweb]

Description:

Expands CI test matrix to include Python 3.9 and 3.10. Python 3.10 is marked as "experimental", so the CI workflow will still succeed even if that job fails.

PR Checklist:

• Code is formatted (run scripts/format)
• Tests pass (run scripts/test)
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[duckontheweb]

This is currently failing because of the bump to orjson==3.5.3. That version does not yet have a wheel for Python 3.10 and building from source requires the nightly version of Rust, which is not configured for this environment. The orjson maintainers seem to be good about creating these wheels (they exists for 3.5.2 on Python 3.10), so we should just wait for those to be available.

[l0b0]

Looks good! How about introducing 3.9 and 3.10 support in separate PRs?

[codecov-commenter]

Codecov Report

Merging #420 (4ee2a5d) into main (46fbaa7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #420   +/-   ##
=======================================
  Coverage   89.76%   89.76%           
=======================================
  Files          40       40           
  Lines        5150     5150           
=======================================
  Hits         4623     4623           
  Misses        527      527           

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[duckontheweb]

How about introducing 3.9 and 3.10 support in separate PRs?

Good call. I updated this PR to focus on 3.9 and will open a separate PR for testing 3.10.