[agl] ensure AGL glyph names are 'str' type

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

Lib/fontTools/agl.py

@@ -731,7 +731,7 @@ def _builddicts():
 		unicode = m.group(1)
 		assert len(unicode) == 4
 		unicode = int(unicode, 16)
-		glyphName = m.group(2)
+		glyphName = tostr(m.group(2))
 		if glyphName in AGL2UV:
 			# the above table contains identical duplicates
 			assert AGL2UV[glyphName] == unicode