[cppyy] fix iadd pythonization for np/array buffers

[aaronj0]

Backport of wlav/CPyCppyy#51
Fixes #18768

Currently, the iadd implementation for std::vector in Pythonize.cxx works perfectly with Python lists and iterables for which we have an ItemGetter. But the code path for numpy/array buffers do not. The difference between x+=y and x.__iadd__(y) leads to unintuitive behaviour: if the intent is to obtain the resulting iterator when calling insert, the user can do d.insert(d.end(), a) which makes more sense than expecting it from d+=a. When the user does call d+=a, since __iadd__ is overriden, operation does not end with the call to VectorIAdd which should just return the iterator, and reflect the change by insertion on d. Instead, it ends with d = d.__iadd__(a) which reassigns d to the returned iterator, losing the vector in the process. This PR fixes this by returning self in our pythonization.

[github-actions]

Test Results

    19 files      19 suites   3d 7h 0m 28s ⏱️
 2 802 tests  2 802 ✅ 0 💤 0 ❌
51 744 runs  51 744 ✅ 0 💤 0 ❌

Results for commit 4d4db8e.

[dpiparo]

Thanks for these changes. Can we add a (some) test(s)?