Alternative approach to #3807 (#4612)

* Alternative approach to #3807 that supports an important PyCLIF use case: `Base` not specified in `classh<Derived>` statement, but passing Derived as Base works anyway.

* NOtest_multiple_inheritance_getattr in test_class_sh_void_ptr_capsule.py (quick experiment)

* Revert "NOtest_multiple_inheritance_getattr in test_class_sh_void_ptr_capsule.py (quick experiment)"

This reverts commit e8f0749b895ad941221316a5fa43d873a2b00397.

* Special handling of pybind11 objects to side-step try_as_void_ptr_capsule_get_pointer __getattr__ issues.

* Inspect `internals.registered_types_py` in `type_is_pybind11_class_()`

* Remove debug code in tests subdir.

* Clean up the modified `try_as_void_ptr_capsule_get_pointer()` implementation and new helper functions.