Traceback (most recent call last):
  File "<string>", line 39, in <module>
AssertionError

According to the traceback, the captured stderr ('err' variable) is an empty string.

The test uses test.support.capture_stderr() which replaces sys.stderr.

The signal module calls PySys_WriteStderr(msg) which calls sys.stderr.write(msg). If the Python call fails, msg is supposed to be written into the C stderr stream. sys.stderr.flush() is not called, but it shouldn't be needed, since test.support.capture_stderr() replace sys.stderr with a io.StringIO object.