WIP: restoring valgrind CI on smart_holder branch (#3090)

* * Rollback of PR #3068.
* Using latest pytest main branch for 3.9 and 3.10.
* WORKAROUND_ENABLING_ROLLBACK_OF_PR3068 in test_class_sh_trampoline_shared_from_this.py

First experiment combining two potential fixes: latest pytest, workaround.
If this succeeds the next step will be to try only latest pytest without the workaround.

Note: the workaround is known to resolve the MSAN error reported under
https://github.com/pybind/pybind11/pull/3068#issuecomment-877658470

* WORKAROUND_ENABLING_ROLLBACK_OF_PR3068 = False

* Narrowing down WORKAROUND_ENABLING_ROLLBACK_OF_PR3068 to Python 3.9