Ensure compatibility with edx-toggles despite upcoming breaking changes

edx-toggles==2.0.0 is likely to suppress namespace objects for the
management of toggle objefcts. We explicitely prevent this by adding a
requirement constraint.

requirements/constraints.txt

@@ -40,6 +40,9 @@ edx-enterprise==3.9.11
 # v2 requires the ES7 upgrade work to be complete
 edx-search<2.0.0
 
+# We expect v2.0.0 to introduce large breaking changes in the feature toggle API
+edx-toggles<2.0.0
+
 # Upgrading to 2.12.0 breaks several test classes due to API changes, need to update our code accordingly
 factory-boy==2.8.1