docstr: Check for a valid shortcut to be displayed.
  
  - GTK will never send 'cmd+' (see `FigureCanvasGTK3._get_key`).
  - The shortcut window only shows keyboard shortcuts, not mouse buttons.
docstr_tok: ["Check","for","a","valid","shortcut","to","be","displayed",".","-","GTK","will","never","send","'cmd+","'","(","see","`","FigureCanvasGTK3._get_key","`",")",".","-","The","shortcut","window","only","shows","keyboard","shortcuts",",","not","mouse","buttons","."]
code: def _is_valid_shortcut(self, key):
      """
      Check for a valid shortcut to be displayed.
  
      - GTK will never send 'cmd+' (see `FigureCanvasGTK3._get_key`).
      - The shortcut window only shows keyboard shortcuts, not mouse buttons.
      """
      return "cmd+" not in key and not key.startswith("MouseButton.")
  
imports: import functools
  import logging
  import os
  from pathlib import Path
  import matplotlib
  from matplotlib import _api, backend_tools, cbook
  from matplotlib.backend_bases import ToolContainerBase, CloseEvent, KeyEvent, LocationEvent, MouseEvent, ResizeEvent
  from gi.repository import Gio, GLib, GObject, Gtk, Gdk
  from .None import _backend_gtk
  from ._backend_gtk import _BackendGTK, _FigureCanvasGTK, _FigureManagerGTK, _NavigationToolbar2GTK, TimerGTK
label_desc: Use image node_id 2 for calling the HelpGTK3 obj's underlying member method code with example usage: obj._is_valid_shortcut(key) and returns: unknown