product_version cannot be set

In build_exe.py2exe.add_versioninfo(),
Version.product_version is hardcoded to version.
Instead, it should default to version if
get("product_version") returns None. To fix this,
simply change the following:

product_version = version

to

product_version = get("product_version") or version

This solution is much better than forcing the user to
work around the problem.