bob limited builds and python versions

[Greg Troxel <gdt%lexort.com@localhost>]

I'm trying to build the set of packages that I need on a machine.  I
generated a pkgpaths file from 'pkgin export' and didn't think hard.

Building, I got e.g. anita built for 311, 312, 313, and 314.  I only
wanted the default version.

Reading https://docs.rs/bob/latest/bob/config/ I don't see how to
express that I only want to use 313 for packages that are multiversion
(or really, only the single version that "make package" would choose;
I'm fine with picking 314 only if that's how it's coded).

I dimly remember that there is some way to do this in pbulk.

How are people dealing with this?