don't include buildtime in watermark

Having the timestamp inside the generated files means that on a rebuild the file changes even if the actual content stays  the same. This is annoying if you want to see what changed on a rebuild and doesn't add all that much, as the timestamp can also be checked via the mtime of the file.
On the other hand, having the cython version in there is potentially useful and doesn't change that often, so leave it in there.