Clean Code: Python

Fix issues found by flake8-bugbear:

* Unused loop variables
* assert statements in non-test code
* mixing test code with production code
* mark one excessive test which takes ~10min to run
  as 'slow'

15_Boxing/python/boxing.py

@@ -48,7 +48,7 @@ def play():
         if opponent_score >= 2 or player_score >= 2:
             break
 
-        for action in range(7):
+        for _action in range(7):
             if random.randint(1, 10) > 5:
                 # opponent swings
                 punch = random.randint(1, 4)