Ufo2ft

Drop kerning pairs with ambiguous horizontal direction (e.g. RTL letters against LTR numerals) (361).

* [postProcessor] Clear existing “post” extraNames.
When renaming glyphs, extraNames attribute of “post” table needs to be cleared as it will contain old names which will otherwise be kept in the table even if unused (360, googlefonts/noto-fonts1543)

OutlineOTFCompiler: Guard against missing defaultWidthX or nominalWidthX (358), in case a UFO sets just one of the two.

- 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.