ci: avoid PySide6 6.12.0 before Python 3.12

PySide6 6.12.0 drops a reference to None on every call to a method that returns nothing (e.g. QPainter.drawEllipse), so None's reference count keeps falling.
Python 3.12+ is unaffected because None is immortal there; on 3.11 the count reaches zero during test_curvebenchmark2, which aborts the job with "Fatal Python error: none_dealloc". PySide6 6.11.2 does not have this bug.

Author: PierreRaybaut

.github/workflows/test-PySide6.yml

@@ -37,7 +37,8 @@ jobs:
         /sbin/start-stop-daemon --start --quiet --pidfile /tmp/custom_xvfb_99.pid --make-pidfile --background --exec /usr/bin/Xvfb -- :99 -screen 0 1920x1200x24 -ac +extension GLX
         python -m pip install --upgrade pip
         python -m pip install ruff pytest
-        pip install PySide6 qtpy numpy
+        # PySide6 6.12.0 drops a reference to None on each void call: fatal before Python 3.12
+        pip install "PySide6!=6.12.0; python_version < '3.12'" "PySide6; python_version >= '3.12'" qtpy numpy
         pip install .
     - name: Lint with Ruff
       run: ruff check --output-format=github qwt