Message 330917 - Python tracker

Hi, I am going to solve this issue through below process.

1. Implement `isAlive` for dummy thread
2. Updating docstrings to recommend use is_alive() instead of isAlive().
3. Add a deprecation warning for both Thread.isAlive and _DummyThread.isAlive
4. Remove isAlive in future Python release (3.8 for the deprecation, 3.8+2 for removal).
5. Backporting the property down to 3.7 and 3.6 doesn't hurt, let's do it

Is it okay?