Add more PLIST_SUB to Mk/Uses/python.mk

• Add PYTHON_PYOEXTENSION and PYTHON_SUFFIX
• Add PYTHON2 and PYTHON3
• Respect PYTHON_VERSION
• Rename PYOEXTENSION to PYTHON_PYOEXTENSION

This change would help:

• Build databases/postgresql*-plpython with Python 3 (It has PLIST issue since bsd.python.mk to Uses/python.mk transition)
• Simplify Makefile

PR: 205807
Differential Revision: https://reviews.FreeBSD.org/D4758
Exp-run by: antoine