Remove mypy errors from python examples

* Either install [Mypy CLI](https://mypy.readthedocs.io/en/stable/) or extension in your IDE of choice.
* Run mypy against all `py/examples` files.
* Refactor where needed, make sure the existing functionality is not broken.

Start by installing the Mypy CLI or an IDE extension, then run mypy against all files under py/examples to identify the reported errors. Refactor the examples as needed while preserving their existing functionality; the work is done when the examples pass mypy without breaking their behavior.

Written by the indexing model from the issue text.