Require glyphsLib>=6.0.0b4: fixes issue with pypy3 and openstep-plist
3.0.0b1
- Require Python 3.7 or above - Added support for Glyphs 3 sources, requires glyphsLib>=6.0.0b3. This has a side-effect of (hopefully temporarily) removing support for pypy3 (see https://github.com/googlefonts/glyphsLib/issues/726)