[PyQt] Const-correctness of sipCpp

[Giovanni Bajo]

Hi Phil,

Within %MethodCode, sipCpp is not const-correct: it always is of
non-const pointer type, even within const methods.

This can cause some subtle bugs, for instance when accessing a method
which has two overloads (a const one, and a slower non-const one, as it
usually happens with containers, for instance).

I believe it would make sense for sipCpp to be a const pointer type when
accessed from within the %MethodCode of a const method.

Thanks!
-- 
Giovanni Bajo
Develer S.r.l.
http://www.develer.com