Explicitly specify encoding

The update to pylint means we were failing because open()
doesn't specify the encoding everywhere.  I have now specified utf-8
everywhere.  This was patchy before, and is default behaviour on the
Pi.  There's a slim chance it will cause some issues on Windows, but
that shouldn't affect anyone outside the developers.

I've also ignored a couple of new spurious warnings, and explicitly
ignored some unused variables.

openflexure_microscope/api/default_extensions/autofocus.py

@@ -245,7 +245,7 @@ def extension_action(args=None):
                 # Run the action
                 return func(self.extension, **arguments)
 
-            def get(self, *args, **kwargs):  # pylint: disable=useless-super-delegation
+            def get(self, *args, **kwargs):  # pylint: disable=useless-super-delegation,arguments-differ
                 # Explicitly wrap the `get` method to allow us to add a docstring
                 return super().get(*args, **kwargs)
 

openflexure_microscope/api/default_extensions/camera_stage_mapping.py

@@ -200,7 +200,7 @@ class CSMExtension(BaseExtension):
             "linear_calibration_y": cal_y,
         }
 
-        with open(CSM_DATAFILE_PATH, "w") as f:
+        with open(CSM_DATAFILE_PATH, "w", encoding="utf-8") as f:
             json.dump(data, f, cls=JSONEncoder)
 
         return data