gh-139504: Catch BrokenPipeError in ssl test_client_sigalgs_mismatch()

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• Issue: test_ssl: test_client_sigalgs_mismatch() fails randomly on macOS with BrokenPipeError #139504

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cc @gpshead @picnixz @encukou

I cannot reproduce the failure on Linux, so I cannot check manually that it does fix #139504 on macOS.

[picnixz]

For now, let's do that. Unfortunately, I don't have a mac so I can't investigate more :( but since the tests are flaky, it's fine (at least, there isn't an issue with the implementation itself I'd say; and if there is, it'll likely be caught in another test as this function doesn't do anything fancy)

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Ok, merged, thanks for the review.