setup.cfg: Register mark for pytest-datafiles

Register the marker for pytest-datafiles. This fixes the following warning that we currently get:

.tox/py37-nocover/lib/python3.7/site-packages/_pytest/mark/structures.py:324
  /Users/csingh43/buildstream/bst-tmp/.tox/py37-nocover/lib/python3.7/site-packages/_pytest/mark/structures.py:324: PytestUnknownMarkWarning: Unknown pytest.mark.datafiles - is this a typo?  You can register custom marks to avoid this warning - for detail
s, see https://docs.pytest.org/en/latest/mark.html
    PytestUnknownMarkWarning,

changed the description

approved this merge request

assigned to @marge-bot123

added 3 commits

• 0568b106...735b005f - 2 commits from branch master
• f4503d15 - setup.cfg: Register mark for pytest-datafiles

Compare with previous version

merged

mentioned in commit aadd6756