We are using unicode_literals in this module since 329261b.
Because of that, on py27 sometimes a TTFont glyphOrder may end up with
a mix of `str` and `unicode` glyph names.
While I'd love to change every single 'text' string to unicode, maybe
in this case it makes sense to use the native `str` type (`bytes` on
py2, unicode string on py3) for the UV2AGL and AGL2UV dictionaries, as
glyph names can only contain ascii characters anyway.
See https://github.com/fonttools/fonttools/pull/774#discussion_r98327429