This adds python3 urllib3 (python3-urllib3 in Debian) to build environment
dependencies. It's the first user of urllib3 in poky, AFAIK. Maybe
documentation could be updated too, e.g.
https://www.yoctoproject.org/docs/latest/mega-manual/mega-manual.html#brief-build-system-packages

On my Debian 9 build container python3-urllib3 wasn't installed by default
or via other dependencies.