Glyphslib

- Ensure instance width class is remapped to user-space values in Glyphs 3 sources (780, 781).
- While importing UFO features.fea to Glyphs, handle the case where a mark class is used as the value for a glyph class (694, 682).
- Fixed regression whereby the advance width of intermediate layers was being overwritten by the width of the associated master layer (773, 774, 777).

* Fix problem building brace layers (741)

- Fix two small problems with localised values (766)
- Fix color on master layer optimization (768)

- Fix reusing master layer glyph when creating COLR v1 table; if master
layer has color attribute, we should use its glyph for the color layer
in the UFO font, but this was not working for COLR v1 layers.

- Make passing --glyph-data to glyph2ufo actually do something.

First stable release in the 6.x series

The major change in this release cycle is the support for Glyphs 3 file format and using openstep-plist for parsing Glyphs file instead of the home-grown plist parser.

This release requires Python >= 3.7.

Other changes since last stable release include:
- Update GlyphData.xml to commit 0fe81a0 (665)
- Implement "Link Metrics ..." custom parameters (641)
- Close open corners with a ufo2ft filter (663)
- Support FontLab 7 Glyphs export format (719)
- Various housekeeping fixes (704, 705, 720)
- Improve `GSNode.read*` performance (732)
- Construct glyph data from unicodes as a fallback (736)
- [builder] Check that vert{Width,Origin} are not None, can be 0 (659)
- [builder] Support hints with target="up" and "down" (613)
- [builder] Store layer names for roundtripping (664)
- [builder] Guard against rogue comments in glyph classes (660)
- [builder] Optionally, only export UFO layers needed for font compilation (674, 722)
- [builder] Respect "Propagate Anchors" custom parameter (683)
- [builder] Only write locations along defined axes (739)
- [builder] Export COLRv0 layers (716)
- [builder] Export COLRv1 layers from Glyphs 3 Color layers (751)
- [builder] Export stylistic set feature names from Glyphs 2 feature notes and Glyphs 3 feature labels (615)
- [builder] Add glyph_data argument to glyphs2ufo and to_ufos() (668)