Another hack for python 3.

It seems there is no longer an ENUM34 compatibility package in in the latest LCG_97python3 release (but still in python 2 version).

Unfortunately the empty ${ENUM34_ROOT} variable has the effect of adding /lib/python2.7/site-packages to the PYTHONPATH, causing the import of the wrong (python 2) system 'enum' package which misses some definitions.