Use shebang compatible with venv

See [Benjamin-Loison/cpython/issues/10](https://github.com/Benjamin-Loison/cpython/issues/10).

datasets/raise/extract_noise.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python
 
 import skimage.restoration
 from skimage import img_as_float