* 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
In OT kern subtable header, the format is the high byte of 'coverage'
bit mask (bits 8-15), and the low byte (bits 0-7) is the actual coverage
bits.
In AAT kern, it's the opposite: the coverage flags are the high byte,
whereas the subtable format is the low byte.
Also adjusted the test data, and set coverage to 1 for OT kern subtable
(which means the usual horizontal kerning).
