The environment markers are presently used directly in nupic’s setup.py to specify a dynamic dependency on pycapnp, which is presently needed on linux and osx, but not desired on windows (see also https://www.python.org/dev/peps/pep-0508/, if curious)
I performed some experimentation in my MacBookPro, and came up with this compatibility matrix for environment markers used in the extras_require property:
setuptools: 20.2.2 +
wheel: 0.24.0 +
pip: 8.1.2 +