Log Message:
scons: Add precise CONFLICTS with pyNN-scons-3*

I had deleted PYTHON_SELF_CONFLICTS because it was overbroad,
resulting in py27-scons-3 incorrectly conflicting with py310-scons-4;
this incorrect conflicts caused incorrect failures during
pkg_rolling-replace and manual building.  However, there is still some
python 3 usage of scons-3, and various scons-3 does conflict.  This
commit adds a manual CONFLICTS line covering all python versions of
scons 3, and excluding other scons major versions.  I believe that now
the declared CONFLICTS are exactly correct, gaining the desired
efficiency while not breaking valid usage.

Thanks to wiz@ for off-list discussion about the subtleties.