When #10482 was introduced, this seems to have broken the installation of any package which depends on a python2 version.

A few examples:
catalyst
grass

@codeandkey is working on a PR for eman2 that requires python2, and we can't quite figure out how to work around this.

It seems like python3 should only be a transient build dependency, but the spec won't allow both python2 as a run dependency and python3 as a build dependency to concretize together.