fix python 3.9 compat

Author: eruvanos

arcade/gui/experimental/focus.py

@@ -110,7 +110,7 @@ class UIFocusMixin(UIWidget):
 
     _focusable_widgets = ListProperty[UIWidget]()
     _focused = Property(0)
-    _interacting: UIWidget | None = None
+    _interacting: Optional[UIWidget] = None
 
     _debug = Property(False)
 
@@ -199,7 +199,7 @@ def _ensure_focused_property(self):
                 else:
                     widget.focused = False
 
-    def _get_focused_widget(self) -> UIWidget | None:
+    def _get_focused_widget(self) -> Optional[UIWidget]:
         if len(self._focusable_widgets) == 0:
             return None
 
@@ -218,7 +218,7 @@ def _walk_widgets(cls, root: UIWidget):
             yield child
             yield from cls._walk_widgets(child)
 
-    def detect_focusable_widgets(self, root: UIWidget | None = None):
+    def detect_focusable_widgets(self, root: Optional[UIWidget] = None):
         """Automatically detect focusable widgets."""
         if root is None:
             root = self