Signal handling fails for certain signals

[generic-ocellot]

When using the criterion climin.stops.OnSignal with signal.SIGUSR1 the module fails with the following message:

Exception TypeError: 'signal handler must be signal.SIG_IGN, signal.SIG_DFL, or a callable object' in <bound method OnSignal.del of <climin.stops.stops.OnSignal object at 0x22b58fd0>> ignored