Add build files for RHEL7.

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Switch requirements based on ${pyver} rather than $(FLAVOR)

The one real difference, importlib, depends on the Python version,
but we have that handy already in the shell snippet; use it.

Signed-off-by: Dan Mick <dan.mick@inktank.com>