ipapython/ipachangeconf.py: change "is not 0" for "!= 0"
    
    Python 3.8 introduced a warning to check for usage of "is not"
    when comparing literals. Any such usage will output:
    SyntaxWarning: "is not" with a literal. Did you mean "!="?
    See: https://bugs.python.org/issue34850
    
    Fixes: https://pagure.io/freeipa/issue/8057
    Signed-off-by: François Cami <fcami@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>