Add __init__.py to page_sets

https://chromium-review.googlesource.com/c/catapult/+/1567486 caused the
Housekeeper-Nightly-RecreateSKPs_Canary bot to fail because it now
expects our page_sets dir to be a python module.

Change-Id: I121bba213ea50dc4ea4139707cd3b5f97c3d7f70
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/209106
Auto-Submit: Ben Wagner aka dogben <benjaminwagner@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Ben Wagner aka dogben <benjaminwagner@google.com>