JSONEncoder's tolist() branch hangs and exhausts memory when a mock reaches a response, instead of raising TypeError

[Macavirus]

This is a follow-up to #7546, which was closed as an unactionable edge case. I hit this while running some tests, memory exploded and my machine ground to a halt.

  import json
  from unittest.mock import Mock
  from rest_framework.utils.encoders import JSONEncoder
  
  json.dumps(Mock(), cls=JSONEncoder)  # never returns, memory grows until OOM

The stdlib json encoder raises TypeError here. DRF's default() sees tolist, calls it, gets another Mock, and repeats forever.

Happens here: https://github.com/encode/django-rest-framework/blob/main/rest_framework/utils/encoders.py#L61-L63