Fix Bug #3: Power battles

This is another subtle bug that crops up frequently in many
different forms. The problem is that this line:

```
player_stats['power'] = player_stats['power'] - enemy_stats['power']
```

PERMANENTLY modifies the value of `player_stats['power']`.
It even modifies the variable *outside* of the function it is
inside. This is why it appears as though our player's power
is slowly draining away.

The fix is to change our code so that it doesn't modify
`player_stats`:

```
power_diff = player_stats['power'] - enemy_stats['power']
```

For much more detail on when and why functions can modify data
outside of themselves, see:

https://robertheaton.com/2014/02/09/pythons-pass-by-object-reference-as-explained-by-philip-k-dick/

editions/6-debugging/3-power-battles.py

@@ -4,9 +4,9 @@ def player_has_enough_power_to_defeat_enemy(player_stats, enemy_stats):
     # Subtract the enemy power from the
     # player power and make sure that
     # the difference is more than 5.
-    player_stats['power'] = player_stats['power'] - enemy_stats['power']
+    power_diff = player_stats['power'] - enemy_stats['power']
 
-    if player_stats['power'] > EXTRA_POWER_NEEDED_TO_DEFEAT_ENEMY:
+    if power_diff > EXTRA_POWER_NEEDED_TO_DEFEAT_ENEMY:
         return True
     else:
         return False