Add __all__ to specify modules which should be imported

doc/ext/notebook.py

@@ -1,3 +1,5 @@
+__all__ = ['python_to_notebook']
+
 import json
 import copy
 

doc/ext/plot2rst.py

@@ -80,7 +80,7 @@ from skimage import io
 from skimage import transform
 from skimage.util.dtype import dtype_range
 
-from notebook import save_ipython_notebook
+from notebook import python_to_notebook
 
 
 LITERALINCLUDE = """