Avoid warning about invalid signals on Python 3.8
Avoid warning about invalid signals on Python 3.8

dbeyer/benchexec,ultimate-pa/benchexec,dbeyer/benchexec,ultimate-pa/benchexec,ultimate-pa/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,sosy-lab/benchexec,sosy-lab/benchexec,sosy-lab/benchexec,dbeyer/benchexec,dbeyer/benchexec,ultimate-pa/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,sosy-lab/benchexec