When installing ase in a pristine pipenv environment with all dependencies specified in the [installation](https://wiki.fysik.dtu.dk/ase/install.html) wiki-page, a number of tests fail with the following error `E fixture 'mocker' not found ` This can be remedied by also installing the 'pytest-mock' module, as outlined in this github issue: https://github.com/pytest-dev/pytest-mock/issues/70 Suggest "pytest-mock" is also specified as an optional dependency. Fix tested and verified on MacOSX Catalina with python 3.9 in a pipenv environment.