Fix Python 3 compatibility by converting cmp-style function to key-st…

…yle function (fixes #149)

core/colors.py

@@ -20,7 +20,7 @@ def read_colors():
     return tuple(sorted(
         [os.path.splitext(os.path.basename(p))[0] for p in
          helpers.get_text_files(paths.get('colors'))],
-        cmp=helpers.sort_underscore_first))
+        key=helpers.key_from_underscore_prefixed_string))
 
 def get_colors(colorscheme=None):
     """

core/helpers.py

@@ -87,11 +87,7 @@ def os_is_64bit():
     """Returns true if running on a 64-bit OS."""
     return platform.machine().endswith('64')
 
-def sort_underscore_first(a, b):
-    """Special sorting function which considers _-prefixed strings to be smaller
-    than all other strings."""
-    if a.startswith('_') == b.startswith('_'):
-        return cmp(a, b)
-    if a.startswith('_'):
-        return -1
-    return 1
+def key_from_underscore_prefixed_string(s):
+    """Converts a string to a key such that strings prefixed with an underscore
+    will be sorted before other strings."""
+    return not s.startswith('_'), s