Change shebang to Python3

Close swcarpentry/styles#271

Makefile

@@ -97,7 +97,7 @@ lesson-check-all :
 
 ## unittest         : run unit tests on checking tools.
 unittest :
-	python bin/test_lesson_check.py
+	@bin/test_lesson_check.py
 
 ## lesson-files     : show expected names of generated files for debugging.
 lesson-files :

bin/lesson_check.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """
 Check lesson files and their contents.

bin/lesson_initialize.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Initialize a newly-created repository."""
 

bin/repo_check.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """
 Check repository settings.

bin/test_lesson_check.py

+#!/usr/bin/env python3
+
 import unittest
 
 import lesson_check

bin/util.py

-
 import sys
 import os
 import json

bin/workshop_check.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 '''Check that a workshop's index.html metadata is valid.  See the
 docstrings on the checking functions for a summary of the checks.