Attached is a quick and dirty patch to prevent the typical user from
being advised to set USE_PYTHON for packages which support both python2
and/or python3, but not 2.7 or 3.2.

If someone can come up with a better fix, by all means send it my way.

For example, see the message produced by dev-python/simplejson-3.0.4-r1.

 * It seems that you need to set USE_PYTHON to make sure that legacy
 * packages will be built with respect to PYTHON_TARGETS correctly:
 *
 *      USE_PYTHON='2.7'
 *
 * Please note that after changing the USE_PYTHON variable, you may need
 * to run 'python-updater' to rebuild affected packages.
 *
 * For more information on python.eclass compatibility, please see
 * the appropriate python-r1 User's Guide chapter [1].
 *
 * [1]
http://www.gentoo.org/proj/en/Python/python-r1/user-guide.xml#doc_chap2