Fix `tox -epackage` to create pex supporting 3.8.
This was missed in but noticed creating 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.
@@ -164,7 +164,7 @@ skip_install = true
commands =
python -m pex -v \
--cache-dir {envtmpdir}/buildcache \
- --interpreter-constraint=">=3.5,<3.8" \
+ --interpreter-constraint=">=3.5,<3.9" \
--interpreter-constraint="==2.7.*" \
--no-compile \
--no-use-system-time \
