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>

examples/radiopalette.py

@@ -21,7 +21,7 @@ box.pack_start(text_view)
 def echo(button, label):
     if not button.props.active:
         return
-    text_view.props.buffer.props.text += "\n" + label
+    text_view.props.buffer.props.text += '\n' + label
 
 # RadioMenuButton