fix shebang typo

Signed-off-by: t0b3 <thomas.bettler@gmail.com>

rdflib/extras/external_graph_libs.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env
+#!/usr/bin/env python
 # encoding: utf-8
 from __future__ import absolute_import
 from __future__ import division