zulip.Client.call_endpoint: Add retry_on_rate_limit_error.

If the call_endpoint method is called with the
"retry_on_rate_limit_error" parameter set to true, wait and retry
automatically on rate limit errors.
See https://chat.zulip.org/#narrow/stream/378-api-design/topic/
Rate.20limits/near/1217048 for the discussion.

zulip/zulip/__init__.py

@@ -697,6 +697,7 @@ def call_endpoint(
         method: str = "POST",
         request: Optional[Dict[str, Any]] = None,
         longpolling: bool = False,
+        retry_on_rate_limit_error: bool = False,
         files: Optional[List[IO[Any]]] = None,
         timeout: Optional[float] = None,
     ) -> Dict[str, Any]:
@@ -707,14 +708,27 @@ def call_endpoint(
             if v is not None:
                 marshalled_request[k] = v
         versioned_url = API_VERSTRING + (url if url is not None else "")
-        return self.do_api_query(
-            marshalled_request,
-            versioned_url,
-            method=method,
-            longpolling=longpolling,
-            files=files,
-            timeout=timeout,
-        )
+
+        while True:
+            result = self.do_api_query(
+                marshalled_request,
+                versioned_url,
+                method=method,
+                longpolling=longpolling,
+                files=files,
+                timeout=timeout,
+            )
+            if not retry_on_rate_limit_error or not (
+                result["result"] == "error"
+                and "code" in result
+                and result["code"] == "RATE_LIMIT_HIT"
+            ):
+                break
+            secs: float = result["retry-after"]
+            logger.warning("hit API rate limit, waiting for %f seconds...", secs)
+            time.sleep(secs)
+
+        return result
 
     def call_on_each_event(
         self,