get OS's python command

def getPyCmd(cls):
        """get OS's python command"""
        if "win32" in sys.platform:
            return 'py'
        elif "linux" in sys.platform:
            return 'python3'
        elif 'darwin' in sys.platform:
            return 'python3'
        else:
            cit.err("No python3 command for " + sys.platform)