Install orjson for core tests

Author: jonmmease

packages/python/plotly/tox.ini

@@ -75,7 +75,7 @@ deps=
     optional: matplotlib==2.2.3
     optional: scikit-image==0.14.4
     optional: kaleido
-    optional: orjson==3.4.6;python_version>"3.5"
+    orjson==3.4.6;python_version>"3.5"
 
 ; CORE ENVIRONMENTS
 [testenv:py27-core]