Adding remaining PYBIND11_SMART_POINTER_HOLDER_TYPE_CASTERS. static_assert for "necessary conditions" for both types of default holder, static_assert for "strict conditions" guarded by new PYBIND11_STRICT_ASSERTS_CLASS_HOLDER_VS_TYPE_CASTER_MIX. All tests build & run as before with unique_ptr as the default holder, all tests build for smart_holder as the default holder, even with the strict static_assert.