Fix Python executable path in uwb-mqtt-publisher

- Updated shebang from '#!/usr/bin/env python' to '#!/usr/bin/env python3'
- Fixes service startup failure on systems without 'python' symlink
- Resolves issue where service exits with status 127

Author: Alex J Lennon

recipes-connectivity/uwb-mqtt-publisher/files/mqtt-live-publisher.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 #
 # Copyright (c) SynchronicIT B.V. 2022. All rights reserved.                           09/08/2022