Fix `tox -epackage` to create pex supporting 3.8. (#843)
Fix `tox -epackage` to create pex supporting 3.8. (#843)

This was missed in #826 but noticed creating #842 where pex.pex startup
was slower than expected due to not using the current (python 3.8)
interpreter and instead proceeding to find all interpreters and then
select the min.
pantsbuild/pex,jsirois/pex,pantsbuild/pex,jsirois/pex