Bugfix/Check actual value when deregistering pybind11 instance (#2252)

* Add tests demonstrating the problem with deregistering pybind11 instances

* Fix deregistering of different pybind11 instance from internals

Co-authored-by: Yannick Jadoul <yannick.jadoul@belgacom.net>
Co-authored-by: Blistic <wots_wot@hotmail.com>