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>
diff --git a/tools/skp/page_sets/__init__.py b/tools/skp/page_sets/__init__.py
new file mode 100644
index 0000000..4a12e35
--- /dev/null
+++ b/tools/skp/page_sets/__init__.py
@@ -0,0 +1,3 @@
+# Copyright 2019 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.