shelf: PySide2-first import with PySide6 fallback for Qt5/6 compatibility

toolbar/hermes_bridge.shelf

@@ -30,6 +30,11 @@ Design notes (why this version differs from the old one):
   making that the default.
 """
 
+try:
+    # Houdini <= 20.5
+    from PySide2 import QtWidgets, QtCore, QtGui
+except ImportError:
+    # Houdini 21.0+
     from PySide6 import QtWidgets, QtCore, QtGui
 import hou
 import socket