Fixed isAlive() references
The 'isAlive()' method was deprecated in Python 3.5 and removed in Python 3.9. I added some quick checks for Python 3.5+ so it can use the correct one for all versions.
The 'isAlive()' method was deprecated in Python 3.5 and removed in Python 3.9. I added some quick checks for Python 3.5+ so it can use the correct one for all versions.