setup.py: enable cross-platform bdist_wheel creation

This works around a distutils bug that incorrectly believes we have
platform-specific code (OS- or CPU-arch- dependent code, like compiled
binary modules), triggered by our lack of modules (of any sort). The bug
prevents "setup.py bdist_wheel --universal" from creating
"versioneer-VER-py2.py3-none-any.whl" bundles, instead creating things
like "versioneer-VER-cp27-none-macoxs_10_8_x86_64.whl".

https://bitbucket.org/pypa/wheel/issue/116/packages-with-only-filesdata_files-get
has more details. Many thanks to @dstufft for tracking this down.