Sonar qube 3 (#166)

* add unittests for mqtt.py

* add mock

* move test requirements into a file

* fix unit tests

* fix formating

* initial version

* fix SonarQube warning

app/src/singleton.py

@@ -1,9 +1,14 @@
+from weakref import WeakValueDictionary
+
+
 class Singleton(type):
-    _instances = {}
+    _instances = WeakValueDictionary()
 
     def __call__(cls, *args, **kwargs):
         # logger_mqtt.debug('singleton: __call__')
         if cls not in cls._instances:
-            cls._instances[cls] = super(Singleton,
-                                        cls).__call__(*args, **kwargs)
+            instance = super(Singleton,
+                             cls).__call__(*args, **kwargs)
+            cls._instances[cls] = instance
+
         return cls._instances[cls]