Default to python3 shebang

vim/UltiSnips/python.snippets

@@ -6,6 +6,9 @@ from __future__ import (division, absolute_import, print_function,
                         unicode_literals)
 endsnippet
 snippet #! "Python shebang" !
+#!/usr/bin/env python3
+endsnippet
+snippet #! "Python2 shebang" !
 #!/usr/bin/env python2
 # encoding: utf-8
 endsnippet