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/AddonManager.py

@@ -37,7 +37,7 @@ import re  # Needed for py 3.6 and earlier, can remove later, search for "re."
 from datetime import date, timedelta
 from typing import Dict, List
 
-from PySide2 import QtGui, QtCore, QtWidgets
+from PySide import QtGui, QtCore, QtWidgets
 import FreeCAD
 import FreeCADGui