Revert "Update auth.py"

Reverts EFForg/OpenWireless#164

Caused a test failure: https://snap-ci.com/EFForg/OpenWireless/branch/master/logs/defaultPipeline/84/python?back_to=build_history

Output:
$ python -m unittest discover -s test/ -p '*_test.py'
..........................E.......
======================================================================
ERROR: test_password (auth_test.TestAuth)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/var/snap-ci/repo/test/auth_test.py", line 59, in test_password
    self.assertTrue(self.auth.is_password("Passw0rd"))
  File "/var/snap-ci/repo/test/../routerapi/auth.py", line 156, in is_password
    if constant_time_equals(hashed, pbkdf2.crypt(candidate, unicode(hashed))):
  File "/var/snap-ci/repo/test/../routerapi/auth.py", line 64, in constant_time_equals
    b = bytearray(b)
TypeError: unicode argument without an encoding