* 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
when no horizontal/vertical header table is present, we assume numberOfHMetrics == maxp.numGlyphs.
This allows to compile/decompile incomplete fonts, only containing head, maxp, hmtx, loca and glyf (or CFF),
that is the bare minimum to temporarily store glyph outline and metrics data for use with sparse/intermediate
master TTFs in varLib.
See https://github.com/googlei18n/ufo2ft/issues/308
