Mk: switch from PYTHON_PORTVERSION to PYTHON_DISTVERSION

This brings python framework in consistense with handbook recommendations
to prefer DISTVERSION and simplifies adding prerelease versions of
python

PR: 255013
Differential Revision: https://reviews.freebsd.org/D29418
Exp-run by: antoine
Approved by: wen@, no objection from python@ or portmgr@