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.

socket.SOCK_STREAM is platform specific and on some
platforms (most notably on Linux on MIPS) does not
equal 1; so it's better to use the constant where
appropriate.

This change fixes the tests on my MIPS32 LE machine.

Signed-off-by: Ivan A. Melnikov <iv@altlinux.org>