Added tests that prove that the thread pool does not work.
Fixed _make_shortcuts to allow the thread pool to work.
Another fix to make the dummy process work.
Copied implementation from CPython. Added the CPython mp tests.
Added a pid property to the thread worker so that the pool will work.
Signals can only be set on the main thread.
Merge branch 'topic/thread-pool' into topic/dill-with-threads