python dependency for build only - ITP required?

[Libor Ukropec via Cygwin-apps]

Hi cygwiners,

I maintain duplicity package that in a new version brought yet another dependency not 
present in cygwin yet (setuptools_scm) but it is needed only just for build (candidate for 
BUILD_REQUIRES).

What is the best approach? Is it even allowed to download the python dependency via pip 
install during the cygport or just propose a new package (ITP)?

Thank you,
Libor