Add test for bad assignments (to self/cls)

[skirpichev]

See this as an example of implementation. This will catch code like

class A:
    def foo(self, arg):
        self = arg

or

class A(object):
    @classmethod
    def foo(cls, arg):
        cls = arg

Yes, this is not a PEP 8 requirement, but there is some wrong if I should introduce new extension for naming tests just for that.

[sigmavirus24]

This plugin is simply for names of functions, methods, and classes. It does not check the bodies for assignments and neither should it. Those are valid assignments in practice although they may introduce unexpected behaviour there is no reason for this plugin to squawk about them.

[skirpichev]

This plugin is simply for names of functions, methods, and classes.

VariablesInFunctionCheck checks also variable names in function body, just like this test.

[sigmavirus24]

I misspoke. Regardless the point stands that this is not a issue of naming but rather something better suggested for a linter not a style checker