Update werkzeug (#2129)

Fix dependabot alert
(https://github.com/svix/svix-webhooks/security/dependabot/182), and
make mypy happy.

Author: svix-mman

python/requirements-dev.txt

@@ -74,7 +74,7 @@ urllib3==2.5.0
     # via
     #   requests
     #   types-requests
-werkzeug==3.0.6
+werkzeug==3.1.4
     # via pytest-httpserver
 wheel==0.44.0
     # via pip-tools

python/tests/test_client.py

@@ -204,7 +204,8 @@ def webhook_handler(request: Request) -> Response:
         assert "Svix-Signature" in request.headers
 
         webhook = Webhook(secret)
-        received_payload = webhook.verify(request.data, request.headers)
+        headers: dict[str, str] = dict(request.headers.items())
+        received_payload = webhook.verify(request.data, headers)
         assert received_payload == payload
 
         return Response("OK")