Use portable python shebangs

* In conda or Gentoo Prefix, we don't want to use the system python and
  instead rely on PATH lookup.

python/setup.py.in

@@ -1,4 +1,4 @@
-#!/usr/bin/python -u
+#!/usr/bin/env python
 #
 # Setup script for libxml2 and libxslt if found
 #