'Bind fun to key-release event of key.
Arguments:
fun -- a function with no arguments
key -- a string: key (e.g. "a") or key-symbol (e.g. "space")
In order to be able to register key-events, TurtleScreen
must have focus. (See method listen.)
Example (for a TurtleScreen instance named screen
and a Turtle instance named turtle):
>>> def f():
fd(50)
lt(60)
>>> screen.onkey(f, "Up")
>>> screen.listen()
### Subsequently the turtle can be moved by
### repeatedly pressing the up-arrow key,
### consequently drawing a hexagon'

def onkey(self, fun, key):

    if (fun is None):
        if (key in self._keys):
            self._keys.remove(key)
    elif (key not in self._keys):
        self._keys.append(key)
    self._onkey(fun, key)
