Python: Add example of models subclassing problem

In reality, we only want to model this as a `rest_framework.response.Response`, since our .qll modeling is more precise for rest-framework responses than if we also modeled it as a basic django http response. (specifically, that default mime-type handling is way different).

Author: RasmusWL

python/ql/test/experimental/library-tests/FindSubclass/Find.expected

@@ -1,6 +1,7 @@
 | Django.Views.View~Subclass | find_subclass_test | Member[clash2] |
 | Django.Views.View~Subclass | find_subclass_test | Member[clash3] |
 | Django.Views.View~Subclass | find_subclass_test | Member[clash] |
+| django.http.response.HttpResponse~Subclass | find_subclass_test | Member[MyRestResponse] |
 | flask.MethodView~Subclass | find_subclass_test | Member[C] |
 | flask.View~Subclass | find_subclass_test | Member[A] |
 | flask.View~Subclass | find_subclass_test | Member[B] |
@@ -14,3 +15,4 @@
 | flask.View~Subclass | find_subclass_test | Member[clash] |
 | flask.View~Subclass | find_subclass_test | Member[complete_module_alias] |
 | flask.View~Subclass | find_subclass_test | Member[complete_module_alias_no_use] |
+| rest_framework.response.Response~Subclass | find_subclass_test | Member[MyRestResponse] |

python/ql/test/experimental/library-tests/FindSubclass/find_subclass_test.py

@@ -51,3 +51,7 @@ class C(flask.views.MethodView):
 
 def wrapper():
     return View # $ MISSING
+
+import rest_framework
+class MyRestResponse(rest_framework.response.Response):
+    pass