Fontmake

- defcon >= 0.5.3

- Added --flatten-components option to flatten nested composite glyphs
so that they have maximum component depth of 1 (714).
- Bumped minimum requirements:
* fonttools >= 4.18.1

- Fixed bug in ``fontmake.instantiator`` when several designspace rules touch the same glyphs (689).
- Update requirements:
* fonttools >= 4.15.0

- Added support for compiling static OTF fonts with `CFF2` table. From the command line you can use the `-o otf-cff2` option.
- Added support for subroutinizing CFF/CFF2 tables using ``cffsubr`` library, based on AFDKO tx tool. This requires ``ufo2ft`` >= 2.15.0 and ``cffsubr`` >= 0.2.6.
By default, ``compreffor`` is still used for subroutinizing OTF with CFF 1.0 tables, and ``cffsubr`` for CFF2, as ``compreffor`` does not support that yet. You can pass ``--subroutinizer cffsubr`` option to explicitly select the latter for either CFF 1 or 2.

Updated requirements:
- fonttools >= 4.12.0

- Use only the default source's glyphs for interpolation (668).
- Update dependencies:
- fontTools: