Fontmake

- glyphsLib >= 3.3.1

- Bumped minimum depedencies:
* fonttools >= 4.50.0
* glyphsLib >= 6.7.0
* ufo2ft >= 3.2.0
- The ``fontmake.instantiator`` was moved to ``ufo2ft.instantiator``, with the same API. It's recommended that you import from the latter. The old module still works but will be removed at some point in the future.
- Fontmake can now build variable fonts with 'sparse' composite glyphs i.e. with either fewer or additional masters than their components' base glyphs, by interpolating missing sources on the fly. This works for both TTF and CFF2 variable fonts (googlefonts/glyphsLib954).

Bump minimum ufo2ft>=3.0.1, fixes a regression introduced in ufo2ft==3.0.0 when compiling a TrueType-flavored variable font with --drop-implied-oncurves option (817).

- Using the new ufo2ft 3.0.0, the feature writers now generate variable FEA that is compiled once per VF, instead of generating/compiling static features for each master and merge OTL tables with fontTools.varLib (provided that the masters do not have different features.fea). You can disable/force the old behavior by passing ``--no-variable-features`` fontmake CLI option (googlefonts/ufo2ft635).
This means fontmake now also supports hand-written variable FEA syntax when compiling variable fonts (see https://github.com/adobe-type-tools/afdko/pull/1350).

- Updated minimum dependencies:
* fonttools >= 4.48.1