this currently raises AttributeError in MutatorMerger.merge for ValueRecord table, because the latter assumes that whenever one of the Device tables are present the respective non-device values are also present, but they may be omitted (and it should default to 0 when missing)
* 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
