sample-apps/layer-python includes __pycache__ folders in the resulting archive in contradiction to AWS docs

The resulting archive contains multiple `__pycache__` subfolders - a practice discouraged by the [AWS docs](https://docs.aws.amazon.com/lambda/latest/dg/python-package.html#python-package-pycache).