fix: attribute error with listeners in cogs

CHANGELOG.md

@@ -31,6 +31,12 @@ These changes are available on the `master` branch, but have not yet been releas
 - Removed `@client.once()` in favour of `@client.listen(once=True)`.
   ([#1957](https://github.com/Pycord-Development/pycord/pull/1957))
 
+### Fixed
+
+- Fixed `AttributeError` caused by
+  [#1957](https://github.com/Pycord-Development/pycord/pull/1957) when using listeners
+  in cogs. ([#1989](https://github.com/Pycord-Development/pycord/pull/1989))
+
 ## [2.4.1] - 2023-03-20
 
 ### Changed

discord/client.py

@@ -442,8 +442,18 @@ def dispatch(self, event: str, *args: Any, **kwargs: Any) -> None:
         # Schedule additional handlers registered with @listen
         for coro in self._event_handlers.get(method, []):
             self._schedule_event(coro, method, *args, **kwargs)
-            if coro._once:
-                once_listeners.append(coro)
+
+            try:
+                if coro._once:  # added using @listen()
+                    once_listeners.append(coro)
+
+            except AttributeError:  # added using @Cog.add_listener()
+                # https://github.com/Pycord-Development/pycord/pull/1989
+                # Although methods are similar to functions, attributes can't be added to them.
+                # This means that we can't add the `_once` attribute in the `add_listener` method
+                # and can only be added using the `@listen` decorator.
+
+                continue
 
         # remove the once listeners
         for coro in once_listeners: