Vim and gVim allow to build against python2 as well as python3 at the same time. For several plugins this is required, because some may only work with python2 and some only work with python3. By restricting the vim and gvim ebuilds to the python-single eclass, the user is prevented to install plugins of both kinds at the same time. Reproducible: Always
If you glance at the history of the (g)vim ebuilds you'll notice this has been toggled back and forth at least once or twice already, mostly recently via bug #615832. We're not going to do it again, especially since python2 is basically at its end of life so plugins requiring it should be updated to python3 or dropped entirely. In fact, newer ebuilds in the tree have dropped python2 support already for exactly this reason.
