ci: Add Python 3.12 to testing to CI as default

[matthewfeickert]

Description

Resolves #2372

• Add Python 3.12 to testing in CI.
• Default to using Python 3.12 in GitHub Actions where possible.
• Add ignore of DeprecationWarning for protobuf
• Add Python 3.12 trove classifier to PyPI metadata.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Add Python 3.12 to testing in CI.
* Default to using Python 3.12 in GitHub Actions where possible.
* Add ignore of DeprecationWarning for protobuf
* Add Python 3.12 trove classifier to PyPI metadata.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.21%. Comparing base (a0d94b9) to head (8aad00c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2460   +/-   ##
=======================================
  Coverage   98.21%   98.21%           
=======================================
  Files          69       69           
  Lines        4541     4541           
  Branches      804      804           
=======================================
  Hits         4460     4460           
  Misses         48       48           
  Partials       33       33           

Flag	Coverage Δ
contrib	97.79% <ø> (ø)
doctest	98.08% <ø> (ø)
unittests-3.10	96.23% <ø> (ø)
unittests-3.11	96.23% <ø> (ø)
unittests-3.12	96.23% <ø> (?)
unittests-3.8	96.25% <ø> (ø)
unittests-3.9	96.27% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.