Pin jupyter-core to < 5.0.0 (#9105)

The recent release of jupyter-core 5.0.0 started emitting a deprecation
warning during the image comparison tests which is causing a failure
caused by usage in seasborn via ipywidgets. To workaround this in the
short term this commit set a constraint on the jupyter-core version. We
should remove this pin after seaborn and ipywidgets are updated, or we
add an explicit warning exclude to the test suite.

constraints.txt

@@ -1,3 +1,8 @@
 # jsonschema pinning needed due nbformat==5.1.3 using deprecated behaviour in
 # 4.0+. The pin can be removed after nbformat is updated.
 jsonschema==3.2.0
+
+# jupyter-core 5.0.0 started emitting deprecation warnings with ipywidgets via
+# a seaborn import. This pin can be removed when compatibility with those
+# packages is fixed
+jupyter-core==4.11.2