Hero sprite must be erased before updating display

[umbe1987]

Before calling pygame.display.update(hero.rect), the hero.rect in the previous frame position should be filled with background, otherwise once it is moved, all its previous positions still get rendered in the next display.update call.
Basically, the solution is explained here:

• Blit a piece of the background over the sprite’s current location, erasing it.
• Append the sprite’s current location rectangle to a list called dirty_rects.
• Move the sprite.
• Draw the sprite at it’s new location.
• Append the sprite’s new location to my dirty_rects list.
• Call display.update(dirty_rects)