Okay, first, the game is a bit sloppy in the idea department. This is basically "Alien Invaders" with limited bullets and with only one map/level. The difficulty choices was good but just increasing the enemy speed will just cause faster endings. In my opinion, some enhancements will make it a better game. Such as:
1. Adding more bullets (i.e. five bullet instead of one)
2. Add enemy bullets to dodge, or maybe space debris to keep the game's intensity balance.
3. Adding mouse control instead of keyboard only will give players more choices of action.
Lastly, you forgot to clean up the game upon closing. Don't forget to add: pygame.quit() after your QUIT event. Without that, the game window will persist until you close the interpreter/console.
Happy coding!