Declare support for Python 3.12

[hugovk]

• Add Trove classifier to declare support for Python 3.12
• Link to changelog from PyPI
• Test PyPy3.9
• Update pre-commit to fix isort/poetry (also triggered Black and pyproject-fmt changes)

[codecov[bot]]

Codecov Report

Merging #231 (d8c8df5) into main (ed5d66c) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #231   +/-   ##
=======================================
  Coverage   94.38%   94.38%           
=======================================
  Files           5        5           
  Lines        2281     2281           
=======================================
  Hits         2153     2153           
  Misses        128      128           

Flag	Coverage Δ
macos-latest	94.34% <100.00%> (+0.08%)	:arrow_up:
ubuntu-latest	94.34% <100.00%> (ø)
windows-latest	94.30% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
tests/test_prettytable.py	100.00% <ø> (ø)
src/prettytable/prettytable.py	90.69% <100.00%> (ø)

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