gh-36895: fix qepcad doctest
This test started to fail on Sage 10.3.beta2 after the merged update of
qepcad to 1.74 in https://github.com/sagemath/sage/pull/36837
URL: https://github.com/sagemath/sage/pull/36895
Reported by: Dima Pasechnik
Reviewer(s): David Coudert
