Incomplete list of valid signal numbers. #1703

Hi,
The list of valid signal number defined in supervisor/datatypes.py is incomplete.
# all valid signal numbers
SIGNUMS = [ getattr(signal, k) for k in dir(signal) if k.startswith('SIG') ]

the numbers between Signals.SIGRTMIN: 34 and Signals.SIGRTMAX: 64 are valid too.

Please add them, thanks