Remove SimpleClient, Producer, Consumer, Unittest

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 * Old partitioners used by `SimpleProducer`; these are superceded by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`.

Author: jeffwidman

README.rst

@@ -151,10 +151,3 @@ testing, probing, and general experimentation. The protocol support is
 leveraged to enable a KafkaClient.check_version() method that
 probes a kafka broker and attempts to identify which version it is running
 (0.8.0 to 2.3+).
-
-Low-level
-*********
-
-Legacy support is maintained for low-level consumer and producer classes,
-SimpleConsumer and SimpleProducer. See
-<https://kafka-python.readthedocs.io/en/master/simple.html?highlight=SimpleProducer> for API details.