NOTES.python2vs3

I've reviewed a diff between the last python2.7 version of SWFK 
and the latest greatest version, and these are my proposed tweaks.

Front matter:
 change version string to 0.7.7-python2.7

Preface:
  remove it, it's all about "this book is only compatible with python 3..."

Chapter 1:
  remove reference to preface which will be deleted

Chapter 9:
  python2 needs 'import Tkinter' instead of 'import tkinter'

Appendix C: 
  sys.stdout.write() does not return number of characters written in python2;
  take out the new little note mentioning it