Enable using cppyy with debug build of Python

[Vipul-Cariappa]

The changes here collects residual exceptions thrown by Python, before calling into Python C API.

Reference: https://github.com/root-project/root/pull/19239

[wlav]

I really don't think straight-up sprinkling PyErr_Clear() b/c the "might" be an error is a good idea. Presumably, the error is there for a reason and even if it's only in debug builds, the error should be handled/collected close to its source.