Add support for Python 3.12 and drop EOL 3.6 and 3.7

[hugovk]

The second Python 3.12 release candidate is out! 🚀

Call to action

We strongly encourage maintainers of third-party Python projects to prepare their projects for 3.12 compatibilities during this phase, and where necessary publish Python 3.12 wheels on PyPI to be ready for the final release of 3.12.0.

See also https://dev.to/hugovk/help-test-python-312-beta-1508/

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Python 3.6 and 3.7 are EOL and no longer receiving security updates (or any updates) from the core Python team.

https://devguide.python.org/versions/

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (0f330b6) 83.28% compared to head (99ed86f) 81.89%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #514      +/-   ##
==========================================
- Coverage   83.28%   81.89%   -1.39%     
==========================================
  Files           4        4              
  Lines         718      674      -44     
  Branches      157      148       -9     
==========================================
- Hits          598      552      -46     
- Misses         95      109      +14     
+ Partials       25       13      -12     

Files	Coverage Δ
cloudpickle/compat.py	100.00% <100.00%> (+30.00%)	⬆️
cloudpickle/cloudpickle_fast.py	91.33% <50.00%> (-0.11%)	⬇️
cloudpickle/cloudpickle.py	72.75% <60.00%> (-3.64%)	⬇️

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[ogrisel]

Thanks for the clean-up. This looks good to me. The only problem is the flake8 diff failure, but it looks unrelated to this. Let me try to investigate.

[ogrisel]

I found more code to clean-up based on the test coverage report. This work is ongoing at # #517.