Thanks for the correction! Unfortunately, it didn’t help to explain the problem, at pythonImportsCheckPhase modules do get imported (simplex_tree is a one of cythonized modules):
pythonImportsCheckPhase
Executing pythonImportsCheckPhase
Check whether the following modules can be imported: gudhi gudhi.simplex_tree
pytestcachePhase
/nix/store/2a5aldh7vkh77vhkm68v48jzd712wm3y-python3.8-gudhi-3.4.1