cleanup: intercept invalid __new__ calls
cleanup: intercept invalid __new__ calls

STD_union cannot be instantiated directly.  __init__ raises an exception,
but so should __new__ since that gets called under Python 3.3.

CantemoInternal/pyxb,balanced/PyXB,jonfoster/pyxb-upstream-mirror,pabigot/pyxb,balanced/PyXB,CantemoInternal/pyxb,jonfoster/pyxb-upstream-mirror,jonfoster/pyxb2,balanced/PyXB,jonfoster/pyxb2,jonfoster/pyxb-upstream-mirror,jonfoster/pyxb2,pabigot/pyxb,CantemoInternal/pyxb