* py-pil: Does not build with Python3.
* Set py-pillow to be the default pil provider
* Update package.py
* Change to comments requested by adamjstewart
* Remove version constraint from extends(), avoid a Spack bug.
- Default scope is versioned with spack and can be overridden by site
or user config.
- Default scope provides sensible default concretization preferences
for all of Spack.
- per-platform concretization scope can be added later (to force a
particular MPI on, e.g., Cray systems)
