tests: fix dependencies missing problem

[mvidalgarcia]

______________________________________________________________________________ ERROR collecting invenio_github/admin.py ______________________________________________________________________________
invenio_github/admin.py:28: in <module>
    from flask_admin.contrib.sqla import ModelView
E   ModuleNotFoundError: No module named 'flask_admin'
_____________________________________________________________________________ ERROR collecting invenio_github/bundles.py _____________________________________________________________________________
invenio_github/bundles.py:26: in <module>
    from flask_assets import Bundle
E   ModuleNotFoundError: No module named 'flask_assets'

And conflicts when requirements-level is min: https://github.com/inveniosoftware/invenio-github/pull/78/checks?check_run_id=1523518871