Improve forbidding field shadowing in model multi-inheritance

[aron45]

This is a ticket to improve #17673 for multi inheritance of grand father

class GrandFather(models.Model):
    column1 = models.IntegerField()


class Father(GrandFather):
    column2 = models.IntegerField()


class Child(Father):
    column1 = models.IntegerField()  # this should fail, but it does pass.

[aron45]

​https://github.com/django/django/pull/4018

[Tim Graham <timograham@…>]

In 4d73303ee974c5f1ea6af984d4578d817825026d:

Fixed #24249 -- Improved field shadowing validation in model multi-inheritance.

[Tim Graham <timograham@…>]

In eb9fbc0b1d6497f20204d9032852a5b4ae8b2466:

[1.8.x] Fixed #24249 -- Improved field shadowing validation in model multi-inheritance.

Backport of 4d73303ee974c5f1ea6af984d4578d817825026d from master