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 * Old partitioners used by `SimpleProducer`; these are superceded by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

Remove unused import, whitespace, etc. No functional changes, just
cleaning it up so the diffs of later changes are cleaner.

The docs for `api_version_auto_timeout_ms` mention setting
`api_version='auto'` but that value has been deprecated for years in
favor of `api_version=None`.

Updating the docs for now, and will remove support for `'auto'` in next
major version bump.

Fix #1633

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Also removed `random_port()` in `test.testutil` because its unused as its been replaced
by the one in `test.fixtures`.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

This finishes the split from `kafka.common` to `kafka.errors`/`kafka.structs`.