py-fonttools: updated to 3.28.0

3.28.0:
- [featureVars] Added experimental module to build FeatureVariations
  tables. Still needs to be hooked up to varLib.build
- [fixedTools] Added otRound to round floats to nearest integer towards
  positive Infinity. This is now used where we deal with visual data like X/Y
  coordinates, advance widths/heights, variation deltas, and similar
- [subset] Improved GSUB closure memoize algorithm.
- [varLib.models] Fixed regression in model resolution
- [feaLib.ast] Fixed error when converting SubtableStatement to string
- [varLib.mutator] Set OS/2.usWeightClass and usWidthClass, and
  post.italicAngle based on the 'wght', 'wdth' and 'slnt' axis values
- [py23/loggingTools] Don't automatically set logging.lastResort handler
  on py27. Moved LastResortLogger to the loggingTools module

(adam)