B105:hardcoded_password_string currently throws an error for string
literal variables, dictionary keys, and comparisons which look like
passwords, but does not create an error for class attributes which look
like passwords.

For example password = "mypassword" and password == "mypassword"
would create an error, but my_object.password = "mypassword",
and my_object.password == "mypassword" would not.
This behavior is unintuitive.

Resolves PyCQA#759