Override %py_build from RHEL not to use %__python2 but %__python
    
    The %py_build macro should respect the global definition of %__python,
    however it hardcodes --executable="%{__python2} in RHEL 7
    and hence it makes it impossible to do the following:
    
        %global __python %__python3
        ...
        %py_build
    
    This should be arguably fixed in RHEL 7, but before it is, it it overridden
    in EPEL 7.
    
    See https://bugzilla.redhat.com/show_bug.cgi?id=1757834