add python 3.12 in setup.py

[tomaszbk]

What does this PR do?

The Lightning v2.4 release notes announce Python 3.12 support. However, the README badge and the PyPI site still list Python 3.11 as the latest supported version. This PR updates the PyPI setup to include Python 3.12, which should also cause the README badge to automatically reflect this change from the next release onward.

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📚 Documentation preview 📚: https://pytorch-lightning--20850.org.readthedocs.build/en/20850/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87%. Comparing base (dd2912a) to head (5fd2f32).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #20850   +/-   ##
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  Coverage      87%      87%           
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  Files         268      268           
  Lines       23368    23368           
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  Hits        20330    20330           
  Misses       3038     3038