Add missing typehint for PermissionOverwrite.pair

Technically inferred but better to be explicit

discord/permissions.py

@@ -673,7 +673,7 @@ class PermissionOverwrite:
         else:
             self._values[key] = value
 
-    def pair(self):
+    def pair(self) -> Tuple[Permissions, Permissions]:
         """Tuple[:class:`Permissions`, :class:`Permissions`]: Returns the (allow, deny) pair from this overwrite."""
 
         allow = Permissions.none()