Make constants immutable

It seems like these should not, by default, be modifiable by the user.

Author: mwcraig

astrowidgets/interface_definition.py

@@ -2,10 +2,10 @@
 from abc import abstractmethod
 
 # Allowed locations for cursor display
-ALLOWED_CURSOR_LOCATIONS = ['top', 'bottom', None]
+ALLOWED_CURSOR_LOCATIONS = ('top', 'bottom', None)
 
 # List of marker names that are for internal use only
-RESERVED_MARKER_SET_NAMES = ['all']
+RESERVED_MARKER_SET_NAMES = ('all',)
 
 __all__ = [
     'ImageViewerInterface',