bug#43298: 27.1; Do font locking for Python 3, not 2

[Per Starbäck]

In GNU Emacs 27.1:

        $ emacs -Q -f /tmp/new.py
        for RET print RET len RET

"print" gets the same colour as "for", that is as a keyword.
I think it should get the same as "len", that is as a builtin
function.

Python.el has a section

          ;; Python 2:
          "print" "exec"

because in Python 2 these two were keywords. But in Python 3 they are built-in
functions.

I think it's overkill to try to determine if the buffer contains Python 2 or 3
and highlight them differently. Using the same fontlocking is good enough,
since it's not a big problem to get these in the wrong colour.
But now when Python 2 is officially discontinued I think it's time to
let it follow Python 3 and get the small inconvenience when editing old
code and not when editing current code.