Message 330280 - Python tracker

Well, to satisfy everybody we need to:
1. Implement `isAlive` for dummy thread
2. Add a deprecation warning for both Thread.isAlive and _DummyThread.isAlive
3. Remove isAlive in future Python release (3.8 for the deprecation, 3.8+2 for removal).
4. Backporting the property down to 3.7 and 3.6 doesn't hurt, let's do it

Did I miss something?