Don't attempt to register signals in non-main thread

[serinamarie]

This helps address #10895
The flaky tests on main leading to: ValueError: signal only works in main thread of the main interpreter are getting out of hand. This PR attempts to fix that by only attempting to registering signal on main thread.

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[zzstoatzz]

@serinamarie hmm - what do you think will happen when the tests that previously failed with signal only works in main thread encounter this and do not end up calling signal.signal / just no op? or do you think we are erroneously getting to that signal.signal call in the first place?

[serinamarie]

@serinamarie hmm - what do you think will happen when the tests that previously failed with signal only works in main thread encounter this and do not end up calling signal.signal / just no op?

Tests that previously intermittently failed on this error would not fail on the error and the tests would still run. It's just that the signal wouldn't be registered in those cases. It doesn't seem to have affected the ability of the tests to run/pass.

or do you think we are erroneously getting to that signal.signal call in the first place?

I don't think it's necessarily erroneous, just that the tests themselves are sometimes run off the main thread and it's an underlying issue with pytest.

[serinamarie]

@abrookins I think there's more to do here but the errors are becoming more prevalent so this is my attempt to get something across the line. But wondering if you have thoughts.

[abrookins]

Let's do it. Whatever the reason we might get here and try to register a signal handler off the main thread, pytest-related or otherwise, it's not going to work, so we may as well prevent it.

[chrisguidry]

Since we've never observed this error anywhere but tests, or had any users report encountering it, it makes sense to me that we'd just quietly skip this.