$ python -m pytest -v --nbval-lax nb.ipynb 
=========================================================================== test session starts ===========================================================================
platform linux -- Python 3.7.6, pytest-5.4.1, py-1.8.1, pluggy-0.12.0 -- /home/sefkw/mc3/envs/celltestsui/bin/python
cachedir: .pytest_cache
metadata: {'Python': '3.7.6', 'Platform': 'Linux-5.4.0-7634-generic-x86_64-with-debian-bullseye-sid', 'Packages': {'pytest': '5.4.1', 'py': '1.8.1', 'pluggy': '0.12.0'}, 'Plugins': {'nbval': '0.9.5', 'xdist': '1.32.0', 'html': '2.1.1', 'metadata': '1.8.0', 'cov': '2.8.1', 'forked': '1.1.2'}}
rootdir: /home/sefkw/code/external/nbval
plugins: nbval-0.9.5, xdist-1.32.0, html-2.1.1, metadata-1.8.0, cov-2.8.1, forked-1.1.2
collected 2 items                                                                                                                                                         

nb::ipynb::Cell 0 PASSED                                                                                                                                            [ 50%]
nb::ipynb::Cell 1 PASSED                                                                                                                                            [100%]

============================================================================ warnings summary =============================================================================
nbval/plugin.py:115
  /home/sefkw/code/external/nbval/nbval/plugin.py:115: PytestDeprecationWarning: direct construction of IPyNbFile has been deprecated, please use IPyNbFile.from_parent
    return IPyNbFile(path, parent)

nbval/plugin.py:312
nbval/plugin.py:312
  /home/sefkw/code/external/nbval/nbval/plugin.py:312: PytestDeprecationWarning: direct construction of IPyNbCell has been deprecated, please use IPyNbCell.from_parent
    cell, options)

nb.ipynb::Cell 0
  /home/sefkw/mc3/envs/celltestsui/lib/python3.7/site-packages/jupyter_client/manager.py:63: DeprecationWarning: KernelManager._kernel_spec_manager_changed is deprecated in traitlets 4.1: use @observe and @unobserve instead.
    def _kernel_spec_manager_changed(self):

-- Docs: https://docs.pytest.org/en/latest/warnings.html
====================================================================== 2 passed, 4 warnings in 0.55s ======================================================================