CI failing on OS X python 3.9

[hyanwong]

From https://github.com/tskit-dev/tskit/pull/1215/checks?check_run_id=1968865308 it seems like we are getting:

tests/test_cli.py:523: 
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_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
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tests/test_cli.py:63: in capture_output
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    func(*args, **kwargs)
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tskit/cli.py:278: in tskit_main
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    set_sigpipe_handler()
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tskit/cli.py:39: in set_sigpipe_handler
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    signal.signal(signal.SIGPIPE, signal.SIG_DFL)
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_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
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134
signalnum = <Signals.SIGPIPE: 13>, handler = <Handlers.SIG_DFL: 0>
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    @_wraps(_signal.signal)
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    def signal(signalnum, handler):
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>       handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler))
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E       ValueError: signal only works in main thread of the main interpreter
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I can't replicate this on my Mac running the latest OS 11.2.1, Python (anaconda installed) 3.9.1.

[hyanwong]

There's a long thread at pytest-dev/pytest-timeout#8 which doesn't seem OS X specific.

[jeromekelleher]

#1217