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)
Staging instance, all changes can be removed at any time
Without it, mypy errors with 'Cannot call function of unknown type' when called from a type-checked function.
Migrated from D7352 (view on Phabricator)