installation: include pytest-black

[slint]

Since we're slowly starting to widely use black for code-formatting we could centralise this dependency here as well.

Note: we also have to remove the dependency from all modules and cookiecutter-invenio-module

[utnapischtim]

i think it is a good idea to have the black dependency within pytest-invenio, but this would create in the near future a lot of work to upgrade every package which is using pytest-invenio. (for the note: It is doable, there are scripts to do it.)

[ntarocco]

pytest-black is in, can we close this?

[slint]

It's in tests extras, not the base install_requires. That means each module that depends on pytest-invenio has to install it separately.

[slint]

I opened #99 with the change. I wouldn't close the issue yet though, since there's some clean-up to do on the dependent modules to remove the pytest-black dependency from their tests extras. Alternative is that we open a separate issue to track the progress of propagating the change.