Run a command, returning its output, or None if it fails.

def call(exe, *argv):
    """
    Run a command, returning its output, or None if it fails.
    """
    exes = which(exe)
    if not exes:
        returnValue(None)
    stdout, stderr, value = yield getProcessOutputAndValue(
        exes[0], argv, env=os.environ.copy()
    )
    if value:
        returnValue(None)
    returnValue(stdout.decode('utf-8').rstrip('\n'))