Log Message:
py-webassets: Update to 2.0.

2.0 (2019-12-20)
    Jump to 2.0 to leave alpha versioning and start doing semver.
    No actual breaking changes.

    - Make the `sass` and `scss` filters compatible with the reference
      compiler
    - Add new `sass_ruby` and `scss_ruby` filters to use the deprecated
      Ruby Sass compiler
    - Update and improve a number of filters.
    - Add a SRI feature (Arvid Norlander).