Fix GaudiPython classes (follows up !1116)

Fix the workaround introduced in 4f05f036 (for the lack of mixed C++/python inheritance) as part of !1116 (merged). There, the iAlgorithm base class was made into a member but the semantics of attribute lookup were not kept backward compatible (and e.g. the .Histos member of iAlgorithm was not accessible).

See related MRs lhcb/Phys!825 (merged) and lhcb/Bender!206 (merged)