Add 3.12 to the python-version list in CI workflow.

[ezio-melotti]

This is a follow-up of #350 that adds back 3.12 to the python-version list.
It should be merged once the following issue is fixed:

• Python 3.12: ‘PyLongObject’ {aka ‘struct _longobject’} has no member named ‘ob_digit’ aio-libs/aiohttp#7229
• wait for a new release of aiohttp (>=3.8.6)

[ezio-melotti]

The issue has been fixed, but the latest release of aiohttp (3.8.5) doesn't include the fix yet. Once a new release is out we can update aiohttp and try again to see if the tests pass on 3.12.

[AlexWaygood]

X-ref aio-libs/aiohttp#7675