> c) Skip the test if the _tkinter thread is present (...)

I opened issue #11998 for the problem with test_signal and the _tkinter module. To get back green buildbots, I commited a workaround:

New changeset 88dca05ed468 by Victor Stinner in branch 'default':
Issue #11998, issue #8407: workaround _tkinter issue in test_signal
http://hg.python.org/cpython/rev/88dca05ed468

> a) Use pthread_kill() to send the signal directly
> to the right thread (...)

I'm still working on this solution to test blocked signals even if _tkinter is loaded.