Change the python-devel BuildRequires to python-libs

We want a BuildRequires for the distutils module, since it's not in
the regular python package (surprise!), but python-libs is where it
actually lives. python-devel is more for Python/C stuff, and we don't
have any of that.
Merge pull request #36 from dashea/master

Change the python-devel BuildRequires to python-libs