style cleanup: prefer ' for strings

Tomeu prefers ' for strings, so let's use it wherever we don't have a good
reason to use ".

Reviewed-by: James Cameron <quozl@laptop.org>
Acked-by: Simon Schampijer <simon@laptop.org>
CC: Aleksey Lim <alsroot@member.fsf.org>

tests/lib/test_mime.py

@@ -80,5 +80,5 @@ class TestMime(unittest.TestCase):
         self.assertEqual(mime_type, 'text/plain')
 
 
-if __name__ == "__main__":
+if __name__ == '__main__':
     unittest.main()