Modernize code a bit

[einarf]

There are things like def update and arcade.start_render() in examples.

Probably a good idea to encourage on_update and clear(). This will break in 2.7.

[pvcraven]

Agreed. I think the text examples probably need to be updated too.

[pvcraven]

I wonder about aliasing arcade.get_window().clear() with arcade.clear(). This is one of the first commands a new programmer will need, and chaining the dot operators before we really explain how that stuff works could be confusing.

[pvcraven]

Another side-issue, this will cause a warning in PyCharm:

def on_update(self, delta_time) :

Because it is expecting this:

def on_update(self, delta_time: float = 1 / 60) -> None:

I don't normally cover default parameters or typing when starting on functions. Open question about the 'method signature' warning, if it will confuse people, or matching the sig will confuse people.