* Support sbix Glyphs with graphicType = "flip"
Since iOS 17.4 beta, Apple introduced the "flip" graphic type for the sbix glyphs to reference the another glyph to be flipped by their render engine. Their use case is for directional emojis.
Example emoji: https://emojipedia.org/woman_walking_facing_right#technical
This is the example of the output XML after this PR:
```
<glyph graphicType="flip" name="u1F3C3.0.M.u27A1" originOffsetX="0" originOffsetY="0">
<ref glyphname="u1F3C3.0.M" />
</glyph>
```
Before, it was just `<hexdata>` with some data in it.
* 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
