Add QtCore.Qt.MouseButton.MidButton alias for Qt6

If QtCore.Qt.MidButton works with QtPy and Qt6, then QtCore.Qt.MouseButton.MidButton is expected to work as well.

Author: PierreRaybaut

qtpy/QtCore.py

@@ -87,6 +87,7 @@
 
     # Alias for MiddleButton removed in PyQt6 but available in PyQt5, PySide2 and PySide6
     Qt.MidButton = Qt.MiddleButton
+    Qt.MouseButton.MidButton = Qt.MiddleButton
 
     # Add removed definition for `Qt.ItemFlags` as an alias of `Qt.ItemFlag`
     # passing as default value 0 in the same way PySide6 6.5+ does.
@@ -142,6 +143,7 @@
     ) = Qt.BackgroundRole
     Qt.TextColorRole = Qt.ItemDataRole.TextColorRole = Qt.ForegroundRole
     Qt.MidButton = Qt.MiddleButton
+    Qt.MouseButton.MidButton = Qt.MiddleButton
 
     # Map DeprecationWarning methods
     QCoreApplication.exec_ = lambda *args, **kwargs: possibly_static_exec(