So stepping back, you can have a

requirements-dev.txt

if you want (just wrap it in a python_requirements() target), but you don't need to - generally it's idiomatic to have a single requirements.txt with everything in it, since Pants will subset that for you. At test time it'll only take the dependencies needed for that test, at pex-building time it'll only bundle what is actually used into the pex, and so on.