urllib dependency is present when using [grpc, cuda] options

[bgedik]

urllib is only used for the http backend based on my look at the code, but the top level requirements.txt list urllib as a dependency. However, it should be only in the requirements_http.txt and not in the top level requirements.txt.

This creates unnecessary conflicts when one only wants to depend on grpc and not http backend. Dependency conflicts like the one reported here: #648