Issue #16400: Add command line option for isolated mode.

Issue #16400: Add command line option for isolated mode.

 -I

    Run Python in isolated mode. This also implies -E and -s. In isolated mode
    sys.path contains neither the script’s directory nor the user’s
    site-packages directory. All PYTHON* environment variables are ignored,
    too. Further restrictions may be imposed to prevent the user from
    injecting malicious code.