Type hinting for `load` is too strict

The type hinting for `load`'s `data` argument is set to `typing.Mapping` which makes sense for the `many=False` case, but for `many=True` case the type of `data` is `list`, which isn't a `Mapping` type.

Start at the load API and inspect the type annotation for its data argument, then trace how the many=True and many=False cases are represented. Update the annotation so both supported input shapes are covered, and add or adjust tests for loading a mapping and a list.

Written by the indexing model from the issue text.