webhooks: Strengthen format_pull_request_event type.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/webhooks/gogs/view.py

@@ -1,7 +1,8 @@
 # vim:fenc=utf-8
-from typing import Any, Callable, Dict, Optional
+from typing import Any, Dict, Optional
 
 from django.http import HttpRequest, HttpResponse
+from typing_extensions import Protocol
 
 from zerver.decorator import webhook_view
 from zerver.lib.exceptions import UnsupportedWebhookEventType
@@ -147,10 +148,15 @@ def api_gogs_webhook(
     )
 
 
+class FormatPullRequestEvent(Protocol):
+    def __call__(self, payload: Dict[str, Any], include_title: bool) -> str:
+        ...
+
+
 def gogs_webhook_main(
     integration_name: str,
     http_header_name: str,
-    format_pull_request_event: Callable[..., Any],
+    format_pull_request_event: FormatPullRequestEvent,
     request: HttpRequest,
     user_profile: UserProfile,
     payload: Dict[str, Any],