Correction for Issue #398

Apologies, I am not able to edit issue #398. Here's how it should have been typed:

Hello,

The following code fails when trying to use the Thesaurus:

>>> from whoosh.lang.wordnet import *
>>> t = Thesaurus.from_filename("wn_s.pl") 
TypeError: startswith first arg must be bytes or a tuple of bytes, not str

Whoosh: 2.5.7 Python: 3.4.1 Operating System: Linux

This may be related to issue #416.

Duplicate of #415.