ensure build is for python2 and buildrequire python2 (#1202578)
    
    We used to get python2 in the buildroot for free via core
    build system deps, but not any more, so buildrequire it
    explicitly. While we're at it, make sure we're building for
    python2 and python2 only, because this is highly unlikely to
    be py3-compatible. Fixes #1202578 - the last build's python
    module wound up in root, because when the code's Makefile
    tried to find the python sitelib path, there was no python
    executable.