Minor Python fixes

[ewanharris]

Description

This PR fixes some minor issues I noticed while updating the openfga/api sha on `main. Splitting out to a separate PR to help review.

They are as follows:

• the ListUsers API differs on the shape of contextual_tuples, rather than an object with a tuple_keys property it just takes the array directly. So we don't need the ContextualTupleKeys type
• read_latest_authorization_model errors if there are no models
• list_users in the example was using the wrong types

References

openfga/python-sdk#147

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected

If you haven't done so yet, we would appreciate it if you could star the OpenFGA repository. :)
It differs from other endpoints in that it isn't an object but just an array directly