Start pybind11v3: Remove all code for `PYBIND11_INTERNALS_VERSION`s `4` and `5` (#5530)

* Start pybind11v3

* Remove all code for PYBIND11_INTERNALS_VERSION 4 and 5

* Fix oversight: pybind11/_version.py needs to be updated as well.

* Add @pytest.mark.skipif("env.GRAALPY", reason="TODO debug segfault")
8 files changed