Make config.PY3K a property
Make config.PY3K a property

That way it gets updated properly when CONDA_PY gets updated.

Fixes #565.

ilastik/conda-build,rmcgibbo/conda-build,ilastik/conda-build,mwcraig/conda-build,mwcraig/conda-build,sandhujasmine/conda-build,dan-blanchard/conda-build,mwcraig/conda-build,ilastik/conda-build,rmcgibbo/conda-build,rmcgibbo/conda-build,sandhujasmine/conda-build,dan-blanchard/conda-build,dan-blanchard/conda-build,frol/conda-build,frol/conda-build,sandhujasmine/conda-build,frol/conda-build