Support for PyQt6/PySide6.

Currently these must be selected via `QT_API=pyqt6`/`QT_API=pyside6`.

Note that I didn't create a separate backend_qt6agg (and
backend_qt6cairo, and mplcairo.qt6...) as it seems preferable to instead
move towards a single backend_qtagg and allow selection of the actual qt
binding via the orthogonal QT_API mechanism.

Most of the work is just handling attributes that moved out of the Qt
namespace.

Author: anntzer