Ensure conf is imported for threads/tx

Import the conf for threads/tx.py to ensure that the
msg_rate_limit_period is defined prior to the conf entry
being referenced.

aprsd/threads/tx.py

@@ -6,6 +6,7 @@ from oslo_config import cfg
 from ratelimiter import RateLimiter
 
 from aprsd import client
+from aprsd import conf  # noqa
 from aprsd import threads as aprsd_threads
 from aprsd.packets import core, tracker