I am tryed to install jupyter notebook for python 3 by running:

spack install --verbose py-jupyter-notebook@5.0.0 ^python@3.6.0~tk~ucs4
==> Error: Invalid spec: 'python@3.6.0~tk~ucs4'. Package python requires version 2.7:2.8, but spec asked for 3.6.0

The reason for this, that node-js depends on python2 to build:

depends_on('python@2.7:2.8', type='build')

As python2 is only required as build depency for node-js is should be possible to build node-js with python2 and use jupyter with python3.
Is it possible to pass this to spack?