NullBooleanField fixer can remove still-used import

[adamchainz]

Python Version

3.11

Django Version

No response

Package Version

1.13.0

Description

For example:

 from django.db.models import BooleanField
-from django.db.models import NullBooleanField

class MyNullBooleanField(NullBooleanField):
    ...

django-upgrade shouldn't break code, so if we can prevent such import removals from happening when their name is used in some form, that would be good.