Make pure-python affiliated packages separate Python 2 and Python 3 builds

For affiliated packages that are pure-python, the ``build`` directory is not subdivided into Python versions, but this causes issues if one installs using Python 2, then using Python 3 without wiping the build directory because 2to3 won't get run. Is there a way to force the build directory to be sub-divided by Python version as it is for when C code is present?

cc @iguananaut