V3.0.0 @dataclass bug fixed and no more callable objcets

Version 3.0.0 @DataClass from attr bug and no more callable objects.

We were using @dataclass decorator from attr, which caused kaiba not to run when used in an environment that did not have attr installed. Thanks to @ChameleonTartu for finding and reporting the bug.

Non-private code has been changed which is why this is a major version. In the future we will make sure to have clearer line between what is private internal kaiba code and what is the interface that users will use.

Internal

• We are now callable object free. From now on, we will only use functions.