Currently these must be selected via `QT_API=pyqt6`/`QT_API=pyside6`.Note that I didn't create a separate backend_qt6agg (andbackend_qt6cairo, and mplcairo.qt6...) as it seems preferable to insteadmove towards a single backend_qtagg and allow selection of the actual qtbinding via the orthogonal QT_API mechanism.Most of the work is just handling attributes that moved out of the Qtnamespace.

Testing all qt bindings actually caught the fact that PySide makes thebackend not thread-safe.Co-authored-by: Elliott Sales de Andrade <quantum.analyst@gmail.com>

Also, PyGObjects built on different Ubuntus are incompatible, so thecache should be keyed by matrix.os (which includes the version number),not runner.os (which doesn't).

We do not exercise all of the code paths that call _enum in thetests.At least PyQt5 5.8 does not support the enums by name so we still need theversion gating.

Perfer to run with Qt6 if available

Just run with what ever version of PyQt we find.

Used to be parameterized, now just marked.  There is no backend fixture.

If pyqt4 is imported the code just above will catch the problem andif pyqt4 is not imported we can rely on `plt.switch_backend` to handlethe fallback between pyqt/pyside versions.