replace deprecated `Threading.isAlive()` method call

[theStack]

The method has been deprecated since Python 3.8 and removed in Python 3.9, leading to the following error if ran with the latter:

Traceback (most recent call last):
 File "./bitcoin-submittx", line 463, in start
   join_all(threads, timeout)
 File "./bitcoin-submittx", line 342, in join _all
   alive += t.isAlive()
AttributeError: 'NodeConn' object has no attribute 'isAlive'

Using Threading.is_alive() fixes this; it should work with any Python version >= 3.5 as it's been there since, with the same functionality as .isAlive().