Add Python 3.12 builds

[wshanks]

The migrator seems confused by another package from a split feedstock that also produces non-Python packages that qiskit-aer depends on not having Python 3.12 builds yet, so this is a manual migration attempt.

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[wshanks]

This was a fun journey so I will write it up for anyone curious:

• We run the qiskit-aer unit tests in the test step because I don't have full confidence in the Aer build configuration (upstream pins to old dependencies, for example). It is a little overkill but the tests don't take too long compared to the build and it is easier just to run (almost) all of them than to pick out just a few to run.
• In Python 3.12.1 there was a breaking change to unittest that broke skipped tests for testtools. There were some tests that required cvxpy to run which we had just been skipping by not including cvxpy, so the Python 3.12 tests were broken.
• I tried pinning to Python 3.12.0 for the tests but it seems like the conda-forge tools do not allow this. 3.12.1 was still installed into the test environment.
• I tried adding cvxpy to the test environment. This broke the pip check test because cvxpy currently lists pybind11 as a dependency but the conda-forge package does include pybind11 as a dependency.
• I tried adding pybind11 as an extra dependency in the test environment. This led to some error in the cvxpy tests. I didn't want to start debugging tests so I decided to go back to avoiding these tests. Later, I realized that upstream pins the version of cvxpy to avoid the errors that had caused the tests to fail. The pinned version is so old that there are not cvxpy packages for recent versions of Python, so running the cvxpy tests is not an option.
• The cvxpy tests were all in one test class, so I added that class to the exclude regular expression of the test runner. I had to fiddle with the syntax so that the regular expression characters didn't get parsed by the Windows shell.