Wemake-python-styleguide: Restrict the maximum number of base classes aka mixins

[sobolevn]

Rule request

Thesis

There are use cases that promote putting too many bases classes for your class:

class SnippetList(mixins.ListModelMixin,
                  mixins.CreateModelMixin,
                  mixins.DetailModelMixin,
                  mixins.DeleteModelMixin,
                  generics.GenericAPIView):

We need to restrict classes with too many base classes as too complex ones.
We also need to provide an option with reasonable default to make this rule configurable.

I guess 3 should be the default value. But I am open to any other suggestions.

Reasoning

It is almost never possible to navigate to the desired method of a parent class when you need it with multiple mixins.

It is hard to understand mro and super calls. Do not overuse this technique.

[sobolevn]

@novikovfred will this issue be interesting to you?

[novikovfred]

@sobolevn ye, I'll do it