Travis CI: Remove unnecessary Python f-strings

These f-strings are not necessary and are breaking Travis CI jobs.
https://travis-ci.org/github/eclipse/paho.mqtt.python/pull_requests

Signed-off-by: Christian Clauss <cclauss@me.com>

Author: cclauss

examples/loop_trio.py

@@ -47,7 +47,7 @@ def on_socket_register_write(self, client, userdata, sock):
         self._event_large_write.set()
 
     def on_socket_unregister_write(self, client, userdata, sock):
-        print(f"finished large write")
+        print("finished large write")
         self._event_large_write = trio.Event()
 
 
@@ -60,12 +60,12 @@ def on_message(self, client, userdata, msg):
         print("Got response with {} bytes".format(len(msg.payload)))
 
     def on_disconnect(self, client, userdata, rc):
-        print(f'Disconnect result {rc}')
+        print('Disconnect result {}'.format(rc))
 
     async def test_write(self, cancel_scope: trio.CancelScope):
         for c in range(3):
             await trio.sleep(5)
-            print(f"Publishing")
+            print("Publishing")
             self.client.publish(topic, b'Hello' * 40000, qos=1)
         cancel_scope.cancel()