Make extra_chars a bytestring for Python 3 support

[kirsle]

Under Python 3, I initially got this error when using this module:

TypeError: a bytes-like object is required, not 'str'

By making the extra_chars a byte string, this module now works on both Python 2 and 3.

[captainkw]

@marekweb Just curious if you could merge this into master? Love this repo.

[marekweb]

Thanks for contributing this @kirsle!