Add maxsize for lru_cache for python3.7

Author: tim-schilling

debug_toolbar/toolbar.py

@@ -157,7 +157,7 @@ def is_toolbar_request(cls, request):
         return resolver_match.namespaces and resolver_match.namespaces[-1] == app_name
 
     @staticmethod
-    @lru_cache
+    @lru_cache(maxsize=128)
     def get_observe_request():
         # If OBSERVE_REQUEST_CALLBACK is a string, which is the recommended
         # setup, resolve it to the corresponding callable.