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]

var/spack/repos/builtin/packages/py-pil/package.py

@@ -37,7 +37,10 @@ class PyPil(Package):
 
     provides('pil')
 
+    # py-pil currently only works with Python2.
+    # If you are using Python 3, try using py-pillow instead.
     extends('python')
+    depends_on('python@1.5.2:2.8')
 
     def install(self, spec, prefix):
         python('setup.py', 'install', '--prefix=%s' % prefix)