Fix safe_eval() of set literals

There was code to support set literals (on Python 2.7 and newer), but it
was buggy: SAFE_NODES.union() doesn't modify SAFE_NODES in place,
instead it returns a new set object that is then silently discarded.

I added a unit test and fixed the code.  I also changed the version
check to use sys.version_tuple instead of a string comparison, for
consistency with the subsequent Python 3.4 version check that I added in
the previous commit.

lib/ansible/template/safe_eval.py

@@ -67,8 +67,8 @@ def safe_eval(expr, locals={}, include_exceptions=False):
     )
 
     # AST node types were expanded after 2.6
-    if not sys.version.startswith('2.6'):
-        SAFE_NODES.union(
+    if sys.version_info[:2] >= (2, 7):
+        SAFE_NODES.update(
             set(
                 (ast.Set,)
             )

test/units/template/test_safe_eval.py

@@ -19,6 +19,7 @@
 from __future__ import (absolute_import, division, print_function)
 __metaclass__ = type
 
+import sys
 from collections import defaultdict
 
 from ansible.compat.tests import unittest
@@ -45,3 +46,6 @@ class TestSafeEval(unittest.TestCase):
             self.assertEqual(safe_eval('[]', locals=locals_vars), [])
             self.assertEqual(safe_eval('{}', locals=locals_vars), {})
 
+    @unittest.skipUnless(sys.version_info[:2] >= (2, 7), "Python 2.6 has no set literals")
+    def test_set_literals(self):
+        self.assertEqual(safe_eval('{0}'), set([0]))