add Python 3.12 support

[dantownsend]

Making sure the tests run for Python 3.12, and it's shown in PyPI.

[codecov-commenter]

Codecov Report

Merging #888 (b25c359) into master (febe5bb) will decrease coverage by 0.76%.
The diff coverage is 66.66%.

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@@            Coverage Diff             @@
##           master     #888      +/-   ##
==========================================
- Coverage   92.11%   91.36%   -0.76%     
==========================================
  Files         107      107              
  Lines        8115     8115              
==========================================
- Hits         7475     7414      -61     
- Misses        640      701      +61     

Files	Coverage Δ
piccolo/apps/migrations/auto/schema_differ.py	97.72% <ø> (ø)
piccolo/querystring.py	92.40% <100.00%> (-1.27%)	⬇️
piccolo/table.py	96.87% <100.00%> (ø)
piccolo/query/mixins.py	92.72% <50.00%> (-0.81%)	⬇️

... and 17 files with indirect coverage changes

[dantownsend]

The CI is failing because of slotscheck - Python 3.12 has changed generics.

[dantownsend]

Asked about slotscheck with Python 3.12 generics here: ariebovenberg/slotscheck#201

For now, will skip it for Python 3.12 until I can figure out what to do properly.