py-pil: Protect against building with Python3. (#1868)

* 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.

etc/spack/defaults/packages.yaml

@@ -19,3 +19,4 @@ packages:
       mpi: [openmpi, mpich]
       blas: [openblas]
       lapack: [openblas]
+      pil: [py-pillow]