DM-11658 removed a seemingly irrelevant request to install alias packages for python3/python2 support. It seems that this is a global action for the python importer and that packages downstream of pex_exceptions were relying on something else installing the aliases.
Since this is a global compatibility change for all packages, I'm now wondering if we aren't better off calling this once in the base package. Calling install_aliases in each package is annoying because the linter gets upset that the command is run before all imports have finished.
I'm sorry I broke the build. It hadn't occurred to me that downstream packages were relying on an earlier package installing this.