[ci] Use setup-python output for pypy cache key

[cdce8p]

Description

All other jobs use the setup-python output already. Furthermore the pypy fix was just release in v6.0.

• Bump actions/setup-python from 5.6.0 to 6.0.0 #2814
• Include python version in PyPy python-version output actions/setup-python#1110

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.31%. Comparing base (31763d8) to head (abf7d77).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2816   +/-   ##
=======================================
  Coverage   93.31%   93.31%           
=======================================
  Files          91       91           
  Lines       11094    11094           
=======================================
  Hits        10352    10352           
  Misses        742      742           

Flag	Coverage Δ
linux	93.17% <ø> (ø)
pypy	93.31% <ø> (ø)
windows	93.29% <ø> (ø)

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