* 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
Previously otlLib was generating None if the values themselves were
empty even if the value format was non-empty. This happened to work
for compiling to binary since the compiler handles Value=None.
But this was confusing varLib.merger module (as in when building
variable fonts from such otlLib-built master GSUB/GPOS tables, without
roundtripping to OTF/TTF binary first), because in varLib.merger,
a None means "this master doesn't provide that info; skip it"; whereas
in a PairPos table a None as generated by otlLib simply meant "all
values are zero", which is different from "this master doesn't
provide this value".
This fixes that, such that ufo2ft can build variable-font without
saving masters to binary.
Part of https://github.com/googlefonts/ufo2ft/pull/486
While False does get the job done, the value is not always treated as
a boolean. It is overloaded so that 1 or greater is True, but more than
1 has a different meaning than 1. Hence usage should always be as an
integer as documented so that it's clear(er) this isn't just an on/off
toggle.
* Add custom table packer/unpacker registry
* fix documentation for getTableClass(): it will never return None
* add unregisterCustomTableClass(), mostly for testing
