[python3/en] A note about common gotcha with mutable defaults

A reminder to be aware of using mutable defaults.
Whether it is a real problem or not depends on exact use-case, but an update fully describing that and updating code with copying list, or defaulting with None and conditional initialization, would probably negatively affect readability of the example.

python3.html.markdown

@@ -805,6 +805,7 @@ class Superhero(Human):
         # add additional class attributes:
         self.fictional = True
         self.movie = movie
+        # be aware of mutable default values, since defaults are shared
         self.superpowers = superpowers
 
         # The "super" function lets you access the parent class's methods