python: Remove meaningless "All rights reserved"
    
    The python extension source does not use this pointless text. Unify the
    module docstring to use the same license blob of its source.
    
    For more info why "All rights reserved" is meaningless, see:
    https://github.com/libguestfs/nbdkit/commit/952ffe0fc7685ea775ed8879bf0b2cb9df3d9044
    
    Signed-off-by: Nir Soffer <nsoffer@redhat.com>