Add compatibility with jwcrypto 0.3.0 and higher
    
    Jwcrypto 0.3.0 has moved the actual set with keys into a dict field 'keys' rather than
    being the set itself.
    We can't use the export function because that exports the private keys by default, and
    there's no way in 0.2.0 to indicate we don't want private keys, and we can't determine
    whether the function will accept this argument.
    
    Signed-off-by: Patrick Uiterwijk <puiterwijk@redhat.com>
    Reviewed-by: Howard Johnson <merlin@merlinthp.org>