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/package_list.py

@@ -29,9 +29,9 @@ import threading
 
 import FreeCAD
 
-from PySide2.QtCore import *
-from PySide2.QtGui import *
-from PySide2.QtWidgets import *
+from PySide.QtCore import *
+from PySide.QtGui import *
+from PySide.QtWidgets import *
 
 from Addon import Addon