Fix setuptools handling in Python packages.

- add setuptools dependencies to packages that need it.
- remove setuptools from packages that do not need it.

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

@@ -34,4 +34,5 @@ class PyEmcee(PythonPackage):
 
     version('2.1.0', 'c6b6fad05c824d40671d4a4fc58dfff7')
 
+    depends_on('py-setuptools', type='build')
     depends_on('py-numpy', type=('build', 'run'))