installation: include `pytest-black`

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`

Start by locating the dependency declarations in pytest-invenio, the other modules, and cookiecutter-invenio-module. Confirm where pytest-black should be centralized, remove the duplicate declarations, and verify that installation and formatting dependencies remain available across the affected modules.

Written by the indexing model from the issue text.