Drop the %{py3dir} pattern
    
    %{py3dir} is outside of the expected build directory,
    causing this error when only building for Python 3:
    
        Empty %files file /builddir/build/BUILD/pyparted-3.11.2/debugsourcefiles.list
    
    That's because the mechanism that looks for sources checks all the compiled
    files in the build directory and because they are all outside, it fails.
    This hasn't happened with the python2 subpackage, because it found the
    compiled python2 files.
    
    Note that %{py3dir} was part of the Python guidelines early in the Python 3
    adoption in Fedora, long before we've introduced the debugsource packages.
    
    Instead, we create a directory called "py3dir" in the build directory.
    There are other solutions possible, but this is the least intrusive in this case.