Uses/python.mk: Fix PYTHON_REL for pre-release Python versions

Modify the PYTHON_REL variable assignment in python.mk so that it
can support PORTVERSION's that have non-integer suffixes such as
the current lang/python35 version (PORTVERSION currently 3.5.0.r3).

Currently, ports that use PYTHON_REL while DEFAULT_VERSIONS is set to
3.5, fail to build with the following make error:

make: "/usr/ports/Mk/Uses/python.mk" line 503:
warning: String comparison operator should be either == or !=
make: "/usr/ports/Mk/Uses/python.mk" line 503:
Malformed conditional (${PYTHON_REL} >= 3200 && defined(_PYTHON_FEATURE_PY3KPLIST))

This is caused by a non-integer value ("r3") in PYTHON_REL.

While I'm here, add a comment block to be clear on what we
(currently) want to achieve.

PR: 203093
Reported by: Andrew Berg (aberg010 my.hennepintech.edu)
Reviewed by: antoine
MFH: 2015Q3
Differential Revision: https://reviews.freebsd.org/D3662