cas.eleves.ens.fr has /serviceValidate, not /p3/serviceValidate, and is thus probably a V2 CAS server. python-cas was broken and using /serviceValidate for V3 while it should have been /p3/serviceValidate, see https://github.com/python-cas/python-cas/commit/c3ac4b6c769fcdf735cc48f9d51707041bc699b2