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/camera/base.py

@@ -54,7 +54,7 @@ class FrameStream(io.BytesIO):
         t: Optional[Type[BaseException]],
         value: Optional[BaseException],
         traceback: Optional[TracebackType],
-    ) -> Optional[bool]:
+    ) -> None:
         self.stop_tracking()
         return super().__exit__(t, value, traceback)