setuptools not always included in virtualenv

[dmreagan]

I'm testing the installation procedures for the review at openjournals/joss-reviews#7120. I tried following the instructions for installing from a git clone.

git clone https://github.com/retostauffer/python-colorspace.git
cd python-colorspace && python setup.py install

It worked just fine from a conda environment, but failed when I tried a virtualenv environment because setuptools was missing. I am on a Ubuntu system. It might be worth adding a line to the documentation that not all virtual environments will include setuptools by default, or else find some way to ensure that setuptools is there.