Add 'font-lock-maximum-decoration' levels for Python
* etc/NEWS: New entry under Python mode.
* lisp/progmodes/python.el (python-font-lock-keywords-level-1)
(python-font-lock-keywords-level-2)
(python-font-lock-keywords-maximum-decoration): New variables
based off the incumbent 'python-font-lock-keywords'.
(python-font-lock-keywords): Change it to a list of the new
symbols, for use in the 'car' of 'font-lock-defaults'.
(python-mode): Set the 'car' of 'font-lock-defaults' to the
value of 'python-font-lock-keywords', instead of the symbol
'python-font-lock-keywords'.