Give test_pr proper env call to python interpreter.

It was already an executable script, but lacking a shebang line
wouldn't work.

tools/test_pr.py

@@ -1,3 +1,4 b''
+#!/usr/bin/env python
 """
 This is a script for testing pull requests for IPython. It merges the pull
 request with current master, installs and tests on all available versions of