unittest.installHandler incorrectly assumes signal.SIGINT will always be set to a callable object, rather than signal.SIG_DFL or signal.SIG_IGN. This breaks if the test is being executed in an environment that e.g. ignores signal.SIGINT, like a bash script that executes 'make' in a subshell, because an attempt to call the original handler (for whatever reason) causes it to try and call an integer.

On top of that, the *tests* for unittest.installHandler in unittest/tests/test_break.py assume not just that signal.SIGINT is set to a callable object, but that they're set to signal.default_int_handler.