Detects the number of cores on a system. Cribbed from pp.

def detect_number_of_cores():
    """
    Detects the number of cores on a system. Cribbed from pp.
    """
    # Linux, Unix and MacOS:
    if hasattr(os, "sysconf"):
        if "SC_NPROCESSORS_ONLN" in os.sysconf_names:
            # Linux & Unix:
            ncpus = os.sysconf("SC_NPROCESSORS_ONLN")
            if isinstance(ncpus, int) and ncpus > 0:
                return ncpus
        else:  # OSX:
            return int(subprocess.check_output(["sysctl", "-n", "hw.ncpu"]))
    # Windows:
    try:
        ncpus = int(os.environ.get("NUMBER_OF_PROCESSORS", ""))
        if ncpus > 0:
            return ncpus
    except ValueError:
        pass
    return 1