backfill: Add typing to RANGE_GENERATORS.
Without it, mypy errors with 'Cannot call function of unknown type' when called from a type-checked function.
Migrated from D7352 (view on Phabricator)
Without it, mypy errors with 'Cannot call function of unknown type' when called from a type-checked function.
Migrated from D7352 (view on Phabricator)