Use PEP 526 type annotations, add some type hints (#26464)

* Add some more type hints to helpers.event

* Change most type comments to variable types

* Remove some superfluous type hints

homeassistant/components/google_assistant/http.py

@@ -93,7 +93,7 @@ class GoogleAssistantView(HomeAssistantView):
 
     async def post(self, request: Request) -> Response:
         """Handle Google Assistant requests."""
-        message = await request.json()  # type: dict
+        message: dict = await request.json()
         result = await async_handle_message(
             request.app["hass"], self.config, request["hass_user"].id, message
         )