docs: def get_open_filename(defaultfilename: str,
                        defaultextension: str,
                        title: str = "Open") -> str:
      """
      Provides a GUI "Open" dialogue (via ``tkinter``) and returns the filename.
      """
      root = tkinter.Tk()  # create and get Tk topmost window
      # (don't do this too early; the command prompt loses focus)
      root.withdraw()  # won't need this; this gets rid of a blank Tk window
      root.attributes('-topmost', True)  # makes the tk window topmost
      filename = filedialog.askopenfilename(
          initialfile=defaultfilename,
          defaultextension=defaultextension,
          parent=root,
          title=title
      )
      root.attributes('-topmost', False)  # stop the tk window being topmost
      return filename