Adding ${python.branch} without the . to Python groups

[Blair Zajac]

I'm working on unifying py2{5,6,7}-logilab-astng and they all depend 
upon the corresponding $py2{5,6,7}-logilab-common.  Can we add to the 
python groups another variable without the . in it, so I can do

depends_lib-append  port:py{python.SOMETHING}-logilab-common

What should we call SOMETHING?

Blair