Make update_css_ranking.py python3 compatible

urllib2 does not exist in python3

Change-Id: I862fe5fcb1f5a65841ad4fdb6885b0788ceed65f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2825453


Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#872384}