Glyphslib

- Improve `GSNode.read*` performance
- Fix italic angle on Glyphs 3
- Fix string escaping in Glyphs 3
- Fix saving quadratic curves in Glyphs 3
- Support Glyphs 3 GSInstance family names
- Support (some) Glyphs 3 alternate layers
- Construct glyph data from unicodes as a fallback
- [builder] Only write locations along defined axes

- [builder] Skip color layers for no export glyphs
- Drop Python 3.6, require >= 3.7
- Require openstep-plist v0.3.0: restores support for pypy3 (726)

- Same as v6.0.0b2, but with a fix to Continuous Deployment

- Export COLR font layers to UFO (716)
- Optionally, only export UFO layers needed for font compilation (674, 722)
- Support FontLab 7 Glyphs export format (719)
- Several fixes to the open corners filter (711, 721)
- Fixes to saving files in Glyphs 3 format (706)
- Allow reading sample text (708), GASP table (709) and vendor ID (707)
from Glyphs 3 fonts
- Various housekeeping fixes (704, 705, 720)

- Implement "Link Metrics ..." custom parameters (641)
- Check that vert{Width,Origin} are not None, can be 0 (659)
- Corner case: guard against rogue comments in glyph classes (660)
- Close open corners with a ufo2ft filter (663)
- Update GlyphData.xml to commit 0fe81a0 (665)
- Support hints with target="up" and "down" (613)
- Store layer names for roundtripping (664)
- [builder] Respect "Propagate Anchors" custom parameter (683)
- Support the Glyphs 3 file format and use openstep-plist for parsing (691)

* Apply master custom params _after_ writing master UFO features
* Roundtrip bracket glyph kerning
* Better error message for when decomposing backgrounds hits non-existant glyph