Support python 3.12

[TGoddessana]

Support Python 3.12

• added tox test environment 3.12
• changed github workflows run test with python3.8 to python3.12
• added classifires in setup.py

Since all the test code passed on python3.12, I just modified the CI to test python3.12.

[lafrech]

Thanks.

Note that apispec and webargs are not tested on 3.12 yet. We need to make the same changes there.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (83d3b72) 99.88% compared to head (a3cf274) 99.88%.
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #591   +/-   ##
=======================================
  Coverage   99.88%   99.88%           
=======================================
  Files          14       14           
  Lines         885      885           
  Branches      192      192           
=======================================
  Hits          884      884           
  Partials        1        1           

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