Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

test_empty_not_allowed_if_allow_empty_is_set_to_false emits deprecation warning in Python 3.9 due to NotImplemented being used in boolean context which always evaluates to True

tests/test_serializer_nested.py::TestNestedSerializerWithMany::test_empty_not_allowed_if_allow_empty_is_set_to_false
  /root/django-rest-framework/rest_framework/exceptions.py:77: DeprecationWarning: NotImplemented should not be used in a boolean context
    return r and self.code == other.code

Expected behavior

Actual behavior