title: Constant class attribute style according to PEP8: uppercase or lowercase?
body: <p>I really like to follow the standard coding style, but can't find an answer on this.</p>
  
  <pre><code>class Card:
      """Card class representing a playing card."""
      RANKS = (None, 'Ace', '2', '3', '4', '5', '6', '7', '8', '9', '10',
               'Jack', 'Queen', 'King')
      SUITS = ('Clubs', 'Spades', 'Diamonds', 'Hearts')
  
      def __init__(self, rank, suit):
          self.rank = rank
          self.suit = suit
  
      def __str__(self):
          return f"{Card.RANKS[self.rank]} of {Card.SUITS[self.suit]}"
  
  
  c = Card(1, 1)
  print(c)
  </code></pre>
  
  <p>Should I write constants in class attributes all_lower_case or ALL_UPPER_CASE?
  PEP8 just says that constants should be ALL_UPPER_CASE at the module level. What about classes?</p>