Решение найдено. Нужно лишь немного изменить while
while True:
gf.check_events(ai_settings, screen, stats, play_button,
ship, aliens, bullets)
if stats.game_active:
ship.update()
gf.update_bullets(ai_settings, screen, ship, aliens, bullets)
gf.update_aliens(ai_settings, stats, screen, ship, aliens, bullets)
gf.update_screen(ai_settings, screen, stats, ship, aliens, bullets, play_button)
# ниже вносимые изменения
else:
gf.update_screen(ai_settings, screen, stats, ship, aliens, bullets, play_button)