Add Python 3.12 to the CI workflow.

[ezio-melotti]

This PR is a follow-up of #590 and adds 3.12 to the CI workflow. 3.12 however is still failing, so we will have to wait until some upstream issues are fixed before merging:

• aiohttp Python 3.12: ‘PyLongObject’ {aka ‘struct _longobject’} has no member named ‘ob_digit’ aio-libs/aiohttp#7229
• yarl Python 3.12: ‘PyLongObject’ {aka ‘struct _longobject’} has no member named ‘ob_digit’ aio-libs/yarl#829
• frozenlist Can't install frozenlist on Python 3.12 aio-libs/frozenlist#433

[codecov]

Codecov Report

Merging #619 (56cf8f0) into main (2c13a8f) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #619   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          356       356           
  Branches        29        29           
=========================================
  Hits           356       356           

Flag	Coverage Δ
Python_3.10	100.00% <ø> (ø)
Python_3.11	100.00% <ø> (ø)
Python_3.9	100.00% <ø> (ø)

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[hugovk]

Still pending 3.12 support from aiohttp: aio-libs/aiohttp#7229