iterkeys is no longer a thing in Python 3

Change-Id: I4d88d2a486db1e552ecac27f123219ac1e03bcaf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2276208
Commit-Queue: Carlos Caballero <carlscab@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>