Use constants from interface definition

Author: mwcraig

astrowidgets/interface_definition.py

@@ -31,10 +31,10 @@ class ImageViewerInterface(Protocol):
     # viewer: Any
 
     # Allowed locations for cursor display
-    ALLOWED_CURSOR_LOCATIONS = ('top', 'bottom', None)
+    ALLOWED_CURSOR_LOCATIONS : tuple = ALLOWED_CURSOR_LOCATIONS
 
     # List of marker names that are for internal use only
-    RESERVED_MARKER_SET_NAMES = ('all', )
+    RESERVED_MARKER_SET_NAMES : tuple = RESERVED_MARKER_SET_NAMES
 
     # The methods, grouped loosely by purpose