mypy: Specify generic type parameters in cachify decorator.

Use of the decorator in event_queue.py suggests concrete return type,
for application of copy() function.

zerver/decorator.py

@@ -75,11 +75,11 @@ def asynchronous(method):
     return wrapper
 
 def cachify(method):
-    # type: (Callable) -> Callable
-    dct = {}  # type: Dict[Tuple, Any]
+    # type: (Callable[..., ReturnT]) -> Callable[..., ReturnT]
+    dct = {}  # type: Dict[Tuple[Any, ...], ReturnT]
 
     def cache_wrapper(*args):
-        # type: (*Any) -> Any
+        # type: (*Any) -> ReturnT
         tup = tuple(args)
         if tup in dct:
             return dct[tup]

zerver/tornado/event_queue.py

@@ -805,7 +805,7 @@ def process_message_event(event_template, users):
 
     @cachify
     def get_client_payload(apply_markdown, client_gravatar):
-        # type: (bool, bool) -> Mapping[str, Any]
+        # type: (bool, bool) -> Dict[str, Any]
         dct = copy.deepcopy(wide_dict)
         MessageDict.finalize_payload(dct, apply_markdown, client_gravatar)
         return dct