Suppress not-an-iterable for model_utils.managers. Fixes #117

[atodorov]

I don't have experience with django-model-utils and I wasn't able to figure out why it produces this error. My search boiled down to IterableChecker._check_iterable inside pylint. After executing infered = safe_infered(node) I get:

1. None when the code under evaluation is a standard Django objects manager
2. Const.NoneType(value=None) when inspecting the .all() of an InheritanceManager

Another possibility is to add a transformation similar to pylint_django/transforms/transforms/django_db_models.py which sets all of the same attributes of InheritanceManager (and friends).

In fact I think I may go ahead with the transformation option instead of this one. @carlio can I inherit the new transformation class from the above one or do I need to copy everything ?

[coveralls]

Pull Request Test Coverage Report for Build 354

• 0 of 0 (NaN%) changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.01%) to 86.606%

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Totals
Change from base Build 350:	-0.01%
Covered Lines:	472
Relevant Lines:	545

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💛 - Coveralls

[atodorov]

@carlio I'm not quire sure how to use the _attribute_is_magic function. Can you take a look?

I'd prefer to actually be able to check the type of the node being one of several known managers instead of blindly disabling the pylint errors when the attribute is named objects.

Note: this same augmentation function will also fix #144. Also notice that we have the not-an-iterable error reported when the objects manager is defined as class attribute regardless of what the actual manager class is.