Ufo2ft

- Dropped support for Python 2.7. Python 3.6 or greater is now required.
- Bumped minimum requirements:
* fonttools >= 4.2.0
* cu2qu >= 1.6.6
* booleanOperations >= 0.9.0
- When computing the glyph bounding boxes, the `ufo2ft.outlineCompiler` now uses the compiled `TTGlyph` or `CFF CharString` objects instead of the UFO glyphs, since the latter may still contain float coordinates and component offsets, which could lead to incorrect off-by-one bounds and sidebearings (356, googlefonts/fontmake593).

- Added ``debugFeatureFile`` option to the compile functions, to dump and inspect the text of auto-generated features (354).
- Added optional ``sortContours`` filter for sorting contours. It works around an issue with non-deterministic contour order from pyclipper across different platforms (345, typemytype/booleanOperations55).

- [outlineCompiler] Glyphs' vertical origin default to OS/2.sTypoAscender instead of hhea.ascent (343)
- [outlineCompiler] Do not require openTypeVheaCaret(Offset|Slope) info attributes to build vhea/vmtx (344).
- Defcon is no longer required explicitly in `setup.py`'s `install_requires`, since ufo2ft never imports from it directly.
The client can still load a UFO using defcon (or ufoLib2) and pass the font object to ufo2ft.

Updated minimum required fonttools to 3.43.0 to incorporate some important bugfixes when building variable fonts.
Read the full fonttools changelog for more info:

- [API] Added compileVariableTTF and compileVariableCFF2 public functions (332).
- [kernFeatureWriter] Added Nandinagari to list of "Indic" scripts (331).
- [preProcessor] Load custom filters in TTFInterpolatablePreProcessor (333).
- Bumped minimum required fonttools >= 3.42.0

- [API] Added compileVariableTTF and compileVariableCFF2 public functions (332).
- [kernFeatureWriter] Added Nandinagari to list of "Indic" scripts (331).
- [preProcessor] Load custom filters in TTFInterpolatablePreProcessor (333).