It's not possible to use `six.with_metaclass` to subclass `models.Model` with a custom metaclass.

[Simon Charette]

A couple of django apps ​in ​the ​wild subclass models.base.BaseModel in order to provide some extra functionality.

If those apps are ever ported to support PY2 and PY3 in the same code base using six they will have to use six.with_metaclass to achieve it.

Unfortunately, models.base.BaseModel.__new__ chokes when a model is defined this way:

from django.db import models
from django.utils import six

class CustomBaseModel(models.base.BaseModel):
    pass

class CustomModel(six.with_metaclass(CustomBaseModel, models.Model)):
    pass

because the 'NewBase' checks don't take into account this case.

[Simon Charette]

All tests pass on Python 2.7.3 SQLite.

[Aymeric Augustin <aymeric.augustin@…>]

In 6b03179e126d4df01623dccc162c1579f349e41e:

Fixed #19688 -- Allow model subclassing with a custom metaclass using six.with_metaclass

[Aymeric Augustin <aymeric.augustin@…>]

In f8b41da43156aa4283c83f2e579e8b18157c9b20:

[1.5.x] Fixed #19688 -- Allow model subclassing with a custom metaclass using six.with_metaclass

Backport of 6b03179e126d4df01623dccc162c1579f349e41e from master.

Although we're post RC 2, I'm backporting this because it's arguably a
major bug in a new feauture that will prevent several well-known
third-party apps from being ported to Python 3.

[Aymeric Augustin <aymeric.augustin@…>]

In 7b49da1c3818cb90f95adaa0b18edaef5f088f16:

Changed testing strategy used in 6b03179e.

Avoid polluting the app cache as it causes unrelated test failures.

Refs #19688.

[Aymeric Augustin <aymeric.augustin@…>]

In 455baa68ebabb472f901e3b476b3344c94090303:

[1.5.x] Changed testing strategy used in 6b03179e.

Avoid polluting the app cache as it causes unrelated test failures.

Refs #19688.

Backport of 7b49da1 from master.