Add Python 3.12 to the test matrix / wheels build

[zerolab]

Changes proposed in this pull request:

• Add Python 3.12 to the mix

The Python team encourages libraries to test against 3.12 now that beta is out

[bigcat88]

thanks for the pull request, I really doubt that everything will be assembled without errors - but it's normal, I'll correct it if anything.
how soon do you need those wheels? What to know - restart the action for this version or can it wait 2-2.5 weeks before the release of the next pillow-heif?

[zerolab]

Thank you for the prompt response @bigcat88
Thought I'd add 3.12 to the Willow test matrix (wagtail/Willow#124) and got the error, hence this PR. I can most certainly wait 2-2.5 weeks.

[bigcat88]

python 3.12 wheels published