Fix quoting of the PyPy entry in the CI test matrix

[fhgffy]

The test matrix has pypy-3.11" without its opening quote, so YAML reads the value as the literal string pypy-3.11". actions/setup-python rejects it:

Invalid 'version' property for PyPy. Both Python version and PyPy versions should satisfy SemVer notation.

The test (ubuntu-latest, pypy-3.11") job therefore fails within seconds on every pull request (for example in #702). The macOS PyPy entry is set separately in include and is unaffected.

This adds the missing quote.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.54%. Comparing base (c4b173a) to head (65dcec1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #703   +/-   ##
=======================================
  Coverage   63.54%   63.54%           
=======================================
  Files          15       15           
  Lines        1270     1270           
=======================================
  Hits          807      807           
  Misses        463      463           

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[agronholm]

Whoops! Thanks!