[3.0.0]: How to build pybind11_tests Python module for running pytest? (before distributing)

[doronbehar]

I'm trying to prepare to the release of pybind11 version 3.0.0rc4 - for distributing it for Nix (see current draft PR). The build process was pretty smooth with our ecosystem. However, from some reason we are getting this error before running pytest:

Traceback (most recent call last):
  File "/build/source/tests/conftest.py", line 25, in <module>
    import pybind11_tests
ModuleNotFoundError: No module named 'pybind11_tests'
ImportError while loading conftest '/build/source/tests/conftest.py'.
tests/conftest.py:25: in <module>
    import pybind11_tests
E   ModuleNotFoundError: No module named 'pybind11_tests'

And I don't understand how am I suppose to create the pybind11_tests directory / module. In our build recipe I tried to run a make pybind11_tests before running pytest, but there doesn't seem to be such a target, although it seems like so from tests/CMakeLists.txt. I did find however tests/pyproject.toml which seems to refer to this pybind11_tests module, but I'm not sure whether I should use it (per the comment on the top of it), or how.