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/tests/test_singleton.py

@@ -0,0 +1,18 @@
+# test_with_pytest.py
+import pytest
+from app.src.singleton import Singleton
+
+class Test(metaclass=Singleton):
+    def __init__(self):
+        pass  # is a dummy test class
+
+def test_singleton_metaclass():
+    a = Test()
+    assert 1 == len(Singleton._instances)
+    b = Test()
+    assert 1 == len(Singleton._instances)
+    assert a is  b
+    del a
+    assert 1 == len(Singleton._instances)
+    del b
+    assert 0 == len(Singleton._instances)