Implement methods to convert a Struct object to a pythonic object

Identity is not the same thing as equality in Python so use ==/!= to compare str, bytes, and int literals. In Python >= 3.8, these instances will raise __SyntaxWarnings__ so it is best to fix them now. https://docs.python.org/3.8/whatsnew/3.8.html#porting-to-python-3-8

% __python__
```
>>> consumer = "cons"
>>> consumer += "umer"
>>> consumer == "consumer"
True
>>> consumer is "consumer"
False
>>> 0 == 0.0
True
>>> 0 is 0.0
False
```

In the 2.0 release, we're removing:
 * `SimpleClient`
 * `SimpleConsumer`
 * `SimpleProducer`
 * Old partitioners used by `SimpleProducer`; these are superceded by
 the `DefaultPartitioner`

These have been deprecated for several years in favor of `KafkaClient`
/ `KafkaConsumer` / `KafkaProducer`.

Since 2.0 allows breaking changes, we are removing the deprecated
classes.

Additionally, since the only usage of `unittest` was in tests for these
old Simple* clients, this also drops `unittest` from the library. All
tests now run under `pytest`.