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/rescue/check_picamera.py

@@ -26,7 +26,7 @@ def main():
 
     try:
         import picamerax
-    except Exception as e:  # pylint: disable=W0703
+    except Exception as _:  # pylint: disable=W0703
         error_sources.append(PICAMERA_IMPORT_ERROR)
     else:
         try:

openflexure_microscope/rescue/check_sangaboard.py

@@ -20,7 +20,7 @@ def main():
             # Try connecting on the specified port
             try:
                 stage = Sangaboard(stage_port)
-            except FileNotFoundError as e:
+            except FileNotFoundError as _:
                 if stage_port:
                     error_sources.append(
                         ErrorSource(