Docs: add tkinter.ttk to Py2/3 cheatsheet

Author: edschofield

docs/compatible_idioms.rst

@@ -1367,6 +1367,7 @@ Tkinter
     import tkFont
     import tkMessageBox
     import tkSimpleDialog
+    import ttk
     
     # Python 2 and 3 (after ``pip install future``):
     import tkinter
@@ -1383,6 +1384,7 @@ Tkinter
     import tkinter.font
     import tkinter.messagebox
     import tkinter.simpledialog
+    import tkinter.ttk
 socketserver
 ~~~~~~~~~~~~
 

docs/notebooks/Writing Python 2-3 compatible code.ipynb

@@ -2944,6 +2944,7 @@
     "import tkFont\n",
     "import tkMessageBox\n",
     "import tkSimpleDialog\n",
+    "import ttk\n",
     "\n",
     "# Python 2 and 3 (after ``pip install future``):\n",
     "import tkinter\n",