[pytest-dev] multiprocessing tests hang on Python3.8+macOS

[Andrew Nelson]

Dear pytest-dev,
on scipy we typically run the test suite using a test script collecting a
series of test arguments, then supplies them to `pytest.main()`.

https://github.com/scipy/scipy/blob/master/runtests.py#L307
then
https://github.com/scipy/scipy/blob/master/scipy/_lib/_testutils.py#L67

I'm finding that all the tests that involve multiprocessing fail, e.g.

```
def test_pool():
    with Pool(2) as p:
        p.map(math.sin, [1,2,3, 4])
```

This is on macOS+Python3.8 that uses 'spawn' as a start method. If I run
the tests directly using pytest there aren't any issues.
Are there any known issues using `pytest.main` with multiprocessing?

Andrew
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