* Bug fix: adding back `!is_alias<Class>(ptr)` that were accidentally omitted.
* Introducing PYBIND11_SH_AVL, PYBIND11_SH_DEF macros. Applying PYBIND11_SH_DEF to test_factory_constructors.py to complete test coverage.
* Using PYBIND11_SH_DEF in test_methods_and_attributes.cpp, for more complete test coverage.
* Using PYBIND11_SH_DEF in test_multiple_inheritance.cpp, for more complete test coverage.
* Cleaning up test_classh_mock.cpp.
* Better explanations for PYBIND11_SH_AVL, PYBIND11_SH_DEF.
* Disabling 3.10-dev builds.
* Using new smart_holder::reclaim_disowned in smart_holder_type_caster for unique_ptr.
* Systematically renaming was_disowned to is_disowned (because disowning is now reversible: reclaim_disowned).
* Systematically renaming virtual_overrider_self_life_support to trampoline_self_life_support (to reuse existing terminology instead of introducing new one).
* Systematically renaming test_class_sh_with_alias to test_class_sh_trampoline_basic.
* Adding a Trampolines and std::unique_ptr section to README_smart_holder.rst.
* MSVC compatibility.
