Or until you trip over the API differences between str and bytes in Python 3 that do not exist in Python 2 (because str == bytes in Python 2). Chiefly, printf-style formatting...

  >>> b"%u" % 5
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  TypeError: unsupported operand type(s) for %: 'bytes' and 'int'