Well it would work just fine, but you'd always end up requiring the external dependency even if you didn't need to.

For example, let's say there was a new urllib released (its still called urllib). It's now version 2.0, but the stdlib version is 1.0.

If your package said "I need urllib==1.0", it would have know way of understanding that the version was already included within the standard library.

That said, it would download the correct package (assuming it existed) and work just fine.