We do the same for feaLib.builder to decide whether to build a Debg table or not: i.e. we check if FONTTOOLS_LOOKUP_DEBUGGING environment variable is defined and if so we go on to build one, otherwise we don't.
Fixes https://github.com/fonttools/fonttools/pull/3018
* 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
