Add 'with' variable instantiation detection to BadNameAttributeUseLinter

[mschwager]

I.e. Context managers.

The BadNameAttributeUseLinter helper currently supports normal variable instantiation of an object, e.g.

def func():
    foo = Foo()
    bar = foo.bad_function()  # Caught by BadNameAttributeUseLinter
    print(bar)

However, the helper does not support instantiation via the with statement, e.g.

def func():
    with Foo() as foo:
        bar = foo.bad_function()  # Not caught by BadNameAttributeUseLinter, yet
        print(bar)

This pattern is not as common as normal variable instantiation, however, it is worth detecting. One of our initial reasons for adding this helper was to catch insecure behavior in tarfile and zipfile, and both of these libraries have a common pattern of instantiation via the with statement.

Let's add context manager variable instantiation support to BadNameAttributeUseLinter.