Type checkers infer the most specific type they can from an assignment. Because it's routine for us to go from Unscheduled to ByTimestamp, we should annotate FORK_CRITERIA with the supertype to avoid a breaking API change.

For example, this:

Should become:

from ethereum.fork_criteria import ByBlockNumber, ForkCriteria

FORK_CRITERIA: ForkCriteria = ByBlockNumber(0)