Misc fixups

• tox.ini: Register pytest mark for datafiles

  This fixes the unknown mark warnings like:

  /bst-plugins-container/.tox/py37/lib/python3.7/site-packages/_pytest/mark/structures.py:332:
  PytestUnknownMarkWarning: Unknown pytest.mark.datafiles - is this a typo?
  You can register custom marks to avoid this warning - for details, see
  https://docs.pytest.org/en/latest/mark.html
      PytestUnknownMarkWarning,

• .gitignore: Fix path to egg-info directory