Improved JSON loading syntax

Minor improvement to file loading: see [PEP 343](https://peps.python.org/pep-0343/) - this syntax adds a built-in `try:`/`finally:` statement around the `json.load()` call, which ensures the file is closed. This is a more robust way to implement the file load, especially valuable for open source code which may be subject to more modifications in the future. Also see [this](https://thispointer.com/python-open-a-file-using-open-with-statement-benefits-explained-with-examples/) page for more practical examples motivating this syntax.

src/its_preselector/test/test_preselector.py

@@ -15,13 +15,10 @@ def setUpClass(cls):
         null_def = json.load(null_file)
         null_file.close()
         cls.empty_preselector = WebRelayPreselector(null_def, {})
-        file = open('sensor_definition.json')
-        sensor_def = json.load(file)
-        file.close()
+        with open('sensor_definition.json', 'r') as f:
+            sensor_def = json.load(f)
         cls.scos_preselector = WebRelayPreselector(sensor_def, {})
 
-
-
     def test_valid_preselector(self):
         self.assertIsNotNone(self.preselector)