Address py38/39 incompatibilities

[dbarbuzzi]

SUMMARY

Handle a few different Python 3.8/3.9 compatibility issues:

• Fix the Python 3.8 compatibility in one of our benchmarking scripts.
• Fix skip for test_local_workers_clean_shutdown (previous skip attempt misused pytest.mark.skipif)
• Skip test_merge_async_iterators (uses Python builtin anext which was added in 3.10)

[dbarbuzzi]

The remote-push recently finished for this PR and py38 had two failures:

1. One of the intermittent marlin failures
   • tests/models/test_marlin.py::test_models[5-32-half-model_pair0]
2. The clean shutdown test failure
   • tests/engine/test_multiproc_workers.py::test_local_workers_clean_shutdown