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.

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.

Conflicts:
	kafka/codec.py
	kafka/version.py
	test/test_producer.py
	test/test_producer_integration.py