Use Factory consistently to create PST instances.  Remove unused mixin.
Use Factory consistently to create PST instances.  Remove unused mixin.

jonfoster/pyxb2,jonfoster/pyxb-upstream-mirror,balanced/PyXB,jonfoster/pyxb-upstream-mirror,pabigot/pyxb,CantemoInternal/pyxb,CantemoInternal/pyxb,balanced/PyXB,pabigot/pyxb,balanced/PyXB,jonfoster/pyxb1,CantemoInternal/pyxb,jonfoster/pyxb2,jonfoster/pyxb1,jonfoster/pyxb2,jonfoster/pyxb-upstream-mirror