Experimental: Guard against Python multiple inheritance divergence

[rwgk]

Description

WIP: Follow-on to PR #4762.

Companion branch: google/pybind11clif#30079

Major things to do:

• Catch all divergence issues in all_type_info_populate() (fix: a long-standing bug in the handling of Python multiple inheritance #4762 (comment)).

• Avoid killing process when divergence is detected:

def NOtest_PPPCCC():
    # terminate called after throwing an instance of 'pybind11::error_already_set'
    # what():  TypeError: bases include diverging derived types:
    #     base=pybind11_tests.python_multiple_inheritance.CppBase,
    #     derived1=pybind11_tests.python_multiple_inheritance.CppDrvd,
    #     derived2=pybind11_tests.python_multiple_inheritance.CppDrvd2
    PPPCCC(11)

• Document advice for how to handle divergence. Roughly (untested): create a C++ derived type (e.g. class CppDrvd1Plus2 : CppDrvd, CppDrvd2 { ... };), wrap with class_<CppDrvd1Plus2>, in Python inherit from CppDrvd1Plus2 instead of CppDrvd, CppDrvd2 separately.

Suggested changelog entry:

[rwgk]

Closing because it is very uncertain when and if work on this will be prioritized. — IMO: Ideally the handling of multiple inheritance in pybind11 is revamped, by porting the Boost.Python approach: https://github.com/boostorg/python/blob/99a5352b5cf790c559a7b976c1ba99520431d9d1/src/object/inheritance.cpp