Log Message:
Update py-webassets to 0.12.1.

0.12.1 (2017-01-08)
    - Fix compatibility with Jinja 2.9.
    - When globbing, include files in alphabetical order (Sam Douglas).
    - Remove duplicate files from bundles (Sam Douglas).
    - Support for PyInstaller (Ilya Kreymer).
    - Fix the sass filter (Dan Callaghan).

0.12 (2016-08-18)
    - Babel filter (JDeuce).
    - NodeSASS filter (Luke Benstead).
    - Autoprefixer 6 filter (Eugeniy Kuznetsov).
    - Many other small changes and improvements by various contributors.