Bump scikit image version for Python 3.8 compatibility

Author: jonmmease

packages/python/plotly/tox.ini

@@ -73,7 +73,8 @@ deps=
     optional: geopandas==0.3.0
     optional: pyshp==1.2.10
     optional: matplotlib==2.2.3
-    optional: scikit-image==0.14.4
+    optional: scikit-image==0.14.4;python_version<"3.0"
+    optional: scikit-image==0.18.1;python_version>"3.5"
     optional: kaleido
     optional: orjson==3.4.6;python_version>"3.5"