Minor cleanups to tox file

Author: dstansby

tox.ini

@@ -13,9 +13,7 @@ extras = testing
 allowlist_externals =
     cp
     ls
-    tree
 commands =
     cp -R {toxinidir}/src/napari_matplotlib/tests/baseline {envdir}/baseline
     ls {toxinidir}/src/napari_matplotlib/tests/baseline
-    python -c 'from skimage import data; data.brain()'
     python -m pytest --mpl --mpl-generate-summary=html --mpl-results-path={toxinidir}/reports -v --color=yes --cov=napari_matplotlib --cov-report=xml