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
Also fixes a bug where glyph alternates in MTI feature files were
wrongly sorted by glyph name. After this change, the output is using
the same ordering as in the input MTI feature file.
Fixes https://github.com/fonttools/fonttools/issues/833.