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Also removed `random_port()` in `test.testutil` because its unused as its been replaced
by the one in `test.fixtures`.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

This commits adds new pytest fixtures in prepation for the
migration of unittest.TestCases to pytest test cases. The handling
of temporary dir creation was also changed so that we can use
the pytest tmpdir fixture after the migration.