Add a test that six was imported.

lib/iris/tests/test_coding_standards.py

@@ -380,6 +380,10 @@ class TestFutureImports(unittest.TestCase):
         r"print_function(,\s*unicode_literals)?\)$",
         flags=re.MULTILINE)
 
+    six_import_pattern = re.compile(
+        r"^import six$",
+        flags=re.MULTILINE)
+
     def test_future_imports(self):
         # Tests that every single Python file includes the appropriate
         # __future__ import to enforce consistent behaviour.
@@ -408,6 +412,12 @@ def test_future_imports(self):
                               'test.'.format(full_fname))
                         failed = True
 
+                    if re.search(self.six_import_pattern, content) is None:
+                        print('The file {} has no valid six import '
+                              'and has not been excluded from the imports '
+                              'test.'.format(full_fname))
+                        failed = True
+
         if failed:
             raise ValueError('There were __future__ import check failures. '
                              'See stdout.')