child classes of Form only include base_fields attributes of parents

[ctdecci@…]

Child classes of Form only include the base_fields attributes of their parent classes, not both, as it should. Here's an example:

from django import newforms as forms
from django.db import models

class ExtendedForm(forms.Form):
    base_fields = { 'new_field': forms.CharField() }

class MyModel(models.Model):
    first_field = models.CharField(maxlength=16)
    second_field = models.CharField(maxlength=16)

obj = MyModel.objects.get(id=1)
MyForm = form_for_instance(obj, form=ExtendedForm)
f = MyForm()

However, when I render f, only new_field shows up.

It appears that the most reasonable place to fix this at the moment is in the DeclarativeFieldsMeta.__new__ method. The method is expecting the fields as class attributes, when instead, the fields are in the attrs dict with the 'base_fields' index. I've attached a patch.

[Chris Beaven]

Closing as a dupe of #5050 which is a bit more comprehensive