Fix an invalid string to bytes comparison in Python 3

Fix an invalid string to bytes comparison in Python 3

In Python 3 this branch would never evaluate to `True` due to comparing a string to bytes.

This was flagged by running some unit tests with the `-bb` flag, so I've added
`-bb` to run with the babel unit tests in Python3.5

Here's some docs on the `-bb` flag:
https://docs.python.org/3.5/library/warnings.html#default-warning-filters
https://docs.python.org/3/howto/pyporting.html#use-continuous-integration-to-stay-compatible