How to run pytest in a developper context?

Environment

Pythonnet version: ce76f2e
Python version: 3.7.9
Operating System: Windows

Details

In the latest master, I find myself unable to run pytest tests. Pretty sure I'm missing something, but I can't find what.

# starting from a clean repo:
python setup.py build
python -m pytest

# pytest output:

==================================ERRORS ==================================
_________________________________________________________________________________________ ERROR collecting test session __________________________________________________________________________________________
C:\Program Files\Python37\lib\importlib\__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
<frozen importlib._bootstrap>:1006: in _gcd_import
    ???
<frozen importlib._bootstrap>:983: in _find_and_load
    ???
<frozen importlib._bootstrap>:967: in _find_and_load_unlocked
    ???
<frozen importlib._bootstrap>:677: in _load_unlocked
    ???
..\env37\lib\site-packages\_pytest\assertion\rewrite.py:171: in exec_module
    exec(co, module.__dict__)
src\tests\conftest.py:26: in <module>
    import clr
E   ModuleNotFoundError: No module named 'clr'
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
==================================1 error in 1.94s ==================================