Ignore assertions from python threading when looking for leaked threads.

While looking for leaked resources (threads) after shutdown and before restart
we in some cases get an assertion in the python threading module where we find
a thread marked as running at the python level but it has no associated thread
at the C level.

homeassistant/__main__.py

@@ -321,10 +321,18 @@ def try_to_restart():
     # Count remaining threads, ideally there should only be one non-daemonized
     # thread left (which is us). Nothing we really do with it, but it might be
     # useful when debugging shutdown/restart issues.
-    nthreads = sum(thread.isAlive() and not thread.isDaemon()
-                   for thread in threading.enumerate())
-    if nthreads > 1:
-        sys.stderr.write("Found {} non-daemonic threads.\n".format(nthreads))
+    try:
+        nthreads = sum(thread.isAlive() and not thread.isDaemon()
+                       for thread in threading.enumerate())
+        if nthreads > 1:
+            sys.stderr.write(
+                "Found {} non-daemonic threads.\n".format(nthreads))
+
+    # Somehow we sometimes seem to trigger an assertion in the python threading
+    # module. It seems we find threads that have no associated OS level thread
+    # which are not marked as stopped at the python level.
+    except AssertionError:
+        sys.stderr.write("Failed to count non-daemonic threads.\n")
 
     # Send terminate signal to all processes in our process group which
     # should be any children that have not themselves changed the process