Added resize

hello_world.py

import pyglet

window = pyglet.window.Window(resizable=True)
label = pyglet.text.Label('Hello, world',
                          font_name='Times New Roman',
                          font_size=36,
                          x=window.width//2, y=window.height//2,
                          anchor_x='center', anchor_y='center')


@window.event
def on_draw():
    window.clear()
    label.draw()

@window.event
def on_resize(w, h):
    label.x = w // 2
    label.y = h // 2

pyglet.app.run()