Incorrect Type Hint for ModelSerializer class `instance` attribute

[JoshFerge]

instance is not a Sequence when used in ModelSerializer code, I believe this type hint was in error.

djangorestframework-stubs/rest_framework-stubs/serializers.pyi

Line 195 in c78da6d

instance: _MT | Sequence[_MT] | None # type: ignore[assignment]

it should just be instance: _MT | None

[JoshFerge]

see #719 (comment)