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 * Old partitioners used by `SimpleProducer`; these are superceded by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

This makes it so the only remaining use of `unittest` is in the old
tests of the deprecated `Simple*` clients. All `KafkaConsumer` tests are
migrated to `pytest`.

I also had to bump the test iterations up on one of the tests, I think there was a race condition there that was more commonly hit under pytest , planning to cleanup that in a followup PR. See https://github.com/dpkp/kafka-python/pull/1886#discussion_r316860737 for details.

Now that we are using `pytest`, there is no need for a custom decorator
because we can use `pytest.mark.skipif()`.

This makes the code significantly simpler. In particular, dropping the
custom `@kafka_versions()` decorator is necessary because it uses
`func.wraps()` which doesn't play nice with `pytest` fixtures:
- https://github.com/pytest-dev/pytest/issues/677
- https://stackoverflow.com/a/19614807/770425

So this is a pre-requisite to migrating some of those tests to using
pytest fixtures.

`random_string` now comes from `test.fixtures` and was being
transparently imported via `test.testutil` so this bypasses the
pointless indirect import.

Similarly, `kafka_version` was transparently imported by `test.testutil`
from `test.fixtures`.

Also removed `random_port()` in `test.testutil` because its unused as its been replaced
by the one in `test.fixtures`.

This is part of the pytest migration that was started back in
a1869c4be5f47b4f6433610249aaf29af4ec95e5.

Use vendored `six`, and also `six.moves.range` rather than `xrange`