Report mutable class attributes as warning

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

The issue does not name files, tests, or an entry point. Start by locating the checks for class attributes and function default arguments in Bandit's Python security rules, then review existing warning tests. Done means mutable class attributes and mutable default parameter values are detected with documented warning behavior, supported by tests.