selectively distinguish real serializers from mocked ones #1006

This addresses an issue where we enter a code path that only
works for real serializers, while the check also allows for objects
covered by OpenApiSerializerExtension.
However when checking for customized ListSerializers, we are only
interested in actual serializers so distinguish the cases.

drf_spectacular/plumbing.py

@@ -101,11 +101,11 @@ def force_instance(serializer_or_field):
         return serializer_or_field
 
 
-def is_serializer(obj) -> bool:
+def is_serializer(obj, strict=False) -> bool:
     from drf_spectacular.serializers import OpenApiSerializerExtension
     return (
         isinstance(force_instance(obj), serializers.BaseSerializer)
-        or bool(OpenApiSerializerExtension.get_match(obj))
+        or (bool(OpenApiSerializerExtension.get_match(obj)) and not strict)
     )
 
 
@@ -119,7 +119,7 @@ def get_list_serializer(obj):
 
 def is_list_serializer_customized(obj) -> bool:
     return (
-        is_serializer(obj)
+        is_serializer(obj, strict=True)
         and get_class(get_list_serializer(obj)).to_representation  # type: ignore
         is not serializers.ListSerializer.to_representation
     )