Temporarily pin to 3.12.0a4 pending dependency support for a5+

[hugovk]

Similar to python/blurb_it#331.

Let's pin to alpha 4 to the CI green and at least have some level 3.12 testing.

Upstream issues:

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

See also notes at python/blurb_it#330 (comment).

[codecov]

Codecov Report

Merging #542 (7ef151d) into main (8fa99c8) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #542   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           18        18           
  Lines         2034      2034           
  Branches       231       231           
=========================================
  Hits          2034      2034           

Flag	Coverage Δ
Python_3.10.10	100.00% <ø> (?)
Python_3.10.9	?
Python_3.11.1	?
Python_3.11.2	100.00% <ø> (?)
Python_3.12.0-alpha.4	100.00% <ø> (ø)
Python_3.8.16	100.00% <ø> (ø)
Python_3.9.16	100.00% <ø> (ø)

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