Don't expose private styles in style.available

They remain in `style.library`, because that's how we look them up, but
this prevents them being exposed as something someone might use.

Fixes itprojects/MasVisGtk#13

Author: QuLogic

lib/matplotlib/style/__init__.py

@@ -246,7 +246,7 @@ def reload_library():
     """Reload the style library."""
     library.clear()
     library.update(_update_user_library(_base_library))
-    available[:] = sorted(library.keys())
+    available[:] = sorted(name for name in library if not name.startswith('_'))
 
 
 reload_library()