docs: def decode_bu64(b):
      """Encode bytes to a URL safe flavor of Base64 used by JWTs.
  
      - Reverse of encode_bu64().
  
      Args:
        b: bytes
          URL safe Base64 encoded bytes to encode.
  
      Returns:
        bytes: Decoded bytes.
  
      """
      s = b
      s = s.replace(b'-', b'+')
      s = s.replace(b'_', b'/')
      p = len(s) % 4
      if p == 0:
          pass
      elif p == 2:
          s += b'=='
      elif p == 3:
          s += b'='
      else:
          raise ValueError('Illegal Base64url string')
      return base64.standard_b64decode(s)