* Replaced all from ...py23 import * with explicit name imports, or removed completely when possible.
* Replaced tounicode() with tostr()
* Changed all BytesIO ans StringIO imports to from io import ..., replaced all UnicodeIO with StringIO.
* Replaced all unichr() with chr()
* Misc minor tweaks and fixes
Before this change, TTX (when running in Python 2.7) would fail
to compile a font if the input contained an `ltag` section.
With Python 3, it worked perfectly fine even before this change.
Resolves https://github.com/behdad/fonttools/issues/357
