Report mutable class attributes as warning

[cleder]

Mutable class attributes may lead to information leaking from one instance to another.

class MyClass(object):
    errors = []

    def do_something(self):
        try:
            ...
        except SomeException as e:
            self.errors.append(e)

Mutable class attributes should be reported as potential security risks.
The logic same should apply for mutable default values of call parameters for functions

[cleder]

Not sure if this should go in here or if PyCQA/flake8-bugbear#57 is the more appropriate place to implement this

[ericwb]

Could you please describe the security issue related to this more. Thanks!