trx_toolkit/clck_gen.py: fix: Thread.isAlive() was removed

As it turns out, in Python < 3.9 class Thread defines 'is_alive'
and 'isAlive = is_alive'.  In Python 3.9 the later has been
removed, so fake_trx.py crashes on receipt of 'POWEROFF':

  File "/home/wmn/wmn/osmocom/bb/src/target/trx_toolkit/clck_gen.py",
    line 63, in running
      return self._thread.isAlive()
  AttributeError: 'Thread' object has no attribute 'isAlive'

See https://bugs.python.org/issue35283 for more details.

Change-Id: Id441d76dddb659958803d507e0fb028fb06422a7

src/target/trx_toolkit/clck_gen.py

@@ -60,7 +60,7 @@ class CLCKGen:
 	def running(self):
 		if self._thread is None:
 			return False
-		return self._thread.isAlive()
+		return self._thread.is_alive()
 
 	def start(self):
 		# Make sure we won't start two threads