fix: Improve file limit test to catch SystemExit or RuntimeError

[esafak]

• Enhance the file limit test to accommodate different exception types. A RedHat user reported a RuntimeError in [self-tests] new test_too_many_open_files fails #2935
• The test now catches SystemExit or RuntimeError.

Fixes #2935

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

[gaborbernat]

I'm holding this until upstream confirms why we're getting RuntimeError and not SystemExit.

[frenzymadness]

I've tried this patch with an update from RuntimeError to OSError but I'm getting:

            # Typically SystemExit, but RuntimeError was observed in RedHat
            expected_exception = typing.Union[SystemExit, OSError]
>           with pytest.raises(expected_exception) as too_many_open_files_exc:
E           TypeError: 'typing.Union' object is not iterable

[esafak]

I've tried this patch with an update from RuntimeError to OSError but I'm getting:

            # Typically SystemExit, but RuntimeError was observed in RedHat
            expected_exception = typing.Union[SystemExit, OSError]
>           with pytest.raises(expected_exception) as too_many_open_files_exc:
E           TypeError: 'typing.Union' object is not iterable

no worries, I'll use a tuple. Let's worry about the exception types. I was able to trigger a RuntimeError in MacOS by commenting out

    # if soft_limit > 1024:
    #     pytest.skip("soft limit for open files is too high to reliably trigger the error")