* 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.