__ipow__ method must ignore its third argument

[cwitty]

Evidently the Python position is that it's OK to pass garbage as the third argument of a .nb_inplace_pow() method; see  http://bugs.python.org/issue1653736 and  http://svn.python.org/view/python/branches/release25-maint/Objects/typeobject.c?r1=53690&r2=53689&pathrev=53690 . In this case, it might be a good idea to forbid the user from writing crashy code.

(I don't know how to trigger the particular call referenced above without the patch on #561, and the next version of that patch will be changed to avoid the problem. So I don't know of an actual test case for this problem.)

[cwitty]

test case that demonstrates segfault

[cwitty]

Actually, I was wrong... the bug has nothing to do with #561. This test case reliably segfaults for me, against current cython-devel. (I still don't think it's a major bug, though, since you have to do something quite strange to trigger it.)

[robertwb]

Merged test case  http://hg.cython.org/cython-devel/rev/999a6d08631d

[dalcinl]

Fixed, and test enabled:

 https://github.com/cython/cython/commit/490fc80e6b494f8ed65f78fc2032d856ec75b8ef  https://github.com/cython/cython/commit/490fc80e6b494f8ed65f78fc2032d856ec75b8ef