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/captures/capture.py

@@ -120,7 +120,8 @@ class CaptureObject(object):
         logging.debug("Finished writing to disk %s", self.file)
 
     def open(self, mode):
-        return open(self.file, mode)
+        # We don't specify an encoding because this will be a binary file.
+        return open(self.file, mode) # pylint: disable=unspecified-encoding
 
     def split_file_path(self, filepath: str):
         """