Addon Manager: Qt6 migration part 1, use PySide

Migrate all uses of "import PySide 2" to use "import PySide" so that the
FreeCAD-provided wrappers can switch between Qt versions.

src/Mod/AddonManager/install_to_toolbar.py

@@ -25,7 +25,7 @@ import os
 
 import FreeCAD
 import FreeCADGui
-from PySide2 import QtCore, QtWidgets
+from PySide import QtCore, QtWidgets
 import Addon
 
 translate = FreeCAD.Qt.translate