Add Python 3.12, retire Python 3.7

[vonschultz]

Is the PR a fix or a feature? Python version upgrade.

Describe the changes in the PR

Assuming we want to support four active Python versions, retire Python 3.7, which is end-of-life since nearly a year ago, and add support for Python 3.12.

New Python versions are released once a year, and make for a good reason to make a release to PyPI, even if there haven't been any major changes to the code base. Not making a release for a year results in warnings such as "An important project maintenance signal to consider for sphobjinv is that it hasn't seen any new versions released to PyPI in the past 12 months, and could be considered as a discontinued project, or that which receives low attention from its maintainers" on https://snyk.io/advisor/python/sphobjinv, which companies use to determine if a project is an acceptable dependency.

Does this PR close any issues?

No.

Does the PR change/update the following, if relevant?

• [x] Documentation
• [x] Tests
• [x] CHANGELOG

[bskinn]

New Python versions are released once a year, and make for a good reason to make a release to PyPI, even if there haven't been any major changes to the code base.

Agreed, I've been meaning to make a release with these bumps for a while but haven't had a chance. Thanks for putting this together! I had it in mind to implement #283 as part of a v2.3.2 that included a Python versions bump like this, but I should probably just do a maintenance release to get it out there.

I already have that linting error addressed in another branch. Once that goes into main and this PR branch is updated, it should resolve.

[bskinn]

Ok, @vonschultz, I've merged the PR with the fix to the flake8 config. Go ahead and update your branch against main, and we'll see how the CI does. You'll probably have to resolve a merge conflict on the CHANGELOG, unfortunately.

[bskinn]

Thanks for this, @vonschultz!