python: Update readme

python: Update readme

* Replace the Python 2 pending removal notice/schedule with a brief
  message about the removal
* Replace mentions of "Python & Python 3" with just "Python" (references
  to the version number are kept only in cases where it is necessary)
* Update Makefile example code to use Python 3 conventions/names

Signed-off-by: Jeffery To <jeffery.to@gmail.com>