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/app.py

@@ -261,10 +261,10 @@ def generate_openapi():
     if fname.endswith(".json"):
         import json
 
-        with open(fname, "w") as fd:
+        with open(fname, "w", encoding="utf-8") as fd:
             json.dump(labthing.spec.to_dict(), fd)
     else:
-        with open(fname, "w") as fd:
+        with open(fname, "w", encoding="utf-8") as fd:
             fd.write(labthing.spec.to_yaml())