I did not go back to check when and why exactly this slipped in.

Also removing unused `msg` (oversight in PR #3059).

* pickle setstate: setattr __dict__ only if not empty, to not force use of py::dynamic_attr() unnecessarily.

* Adding unit test.

* Clang 3.6 & 3.7 compatibility.

* PyPy compatibility.

* Minor iwyu fix, additional comment.

* Addressing reviewer requests.

* Applying clang-tidy suggested fixes.

* Adding check_dynamic_cast_SimpleCppDerived, related to issue #3062.

Adding test_multiple_registered_instances_for_same_pointee_leak. Subtle changes in smart_holder_type_caster_load, trying to work on weak_ptr for shared_ptr_trampoline_self_life_support, but that didn't work out. Manually fully leak-checked again.

Completes unit test coverage for the changed code in smart_holder_type_casters.h.

https://github.com/deepmind/open_spiel/tree/bbfb0259b5d26c1e3f05000f1f4a05b7ad60393e/open_spiel/python/mfg/algorithms
    best_response_value_test
    distribution_test
    fictitious_play_test
    greedy_policy_test
    nash_conv_test
    policy_value_test

CAVEAT: The fix is NOT covered by pybind11 unit tests.

Revert "Updating Catch version to latest, in hopes of resolving the GHA download issue affecting all MSVC builds."

This reverts commit f4e1112dc434d0505cecad6259113f0621ecba70.

It didn't help.

Updating Catch version to latest, in hopes of resolving the GHA download issue affecting all MSVC builds.

Resolving TODOs for `from_unique_ptr` `void_cast_raw_ptr`. Adding test to exercise the path through `cast`. The path through init.h is missing a test that would fail if the flag is incorrect. The plan is to systematically cover all situations (there are many that are not exercised for shared_from_this).

Use casting to `void *` to evade to shared_from_this mechanism only if `pointee_depends_on_holder_owner`, but currently only for `from_raw_ptr_take_ownership`.

Simpler and safe implementation based on new requirement: C++ (outside of the Python object) needs to hold a shared_ptr before shared_from_this is called. This is more similar to the existing implementation on current smart_holder HEAD, but still uses a weak_ptr to always return the same shared_ptr as long as C++ does not let it expire.

smart_holder_poc.h: type-erase raw_ptr before passing to shared_ptr::reset, to not trigger populating the shared_from_this weak_ptr. This way the only way the weak_ptr is populated is through loaded_as_shared_ptr. Breaks all but one test in test_class_sh_trampoline_shared_from_this.py, now marked skip WIP to test everything else with the GitHub CI.

The placeholder `vptr` is never exposed anymore, therefore the externally visible `use_count`s are more intuitive.

Adding `test_pass_released_shared_ptr_as_unique_ptr`, exercising new guard in smart_holder_type_casters.h.

Now also passes the open_spiel iterated_prisoners_dilemma_test ASAN clean, in addition to all pybind11 and PyCLIF unit tests.

The problem was that calling `std::shared_ptr<void>::reset()` with a `void` pointer cannot possibly update the `shared_from_this` `weak_ptr`.

The solution is to store a `shd_ptr_reset` function pointer in `guarded_deleter` (similar in idea to the stored function pointer for calling `delete`).

This commit still includes all debugging code, i.e. is "dirty". The code will be cleaned up after the GitHub CI is fully successful.