replace setup.py with pyproject.toml in packaging.md

[swathipil]

The packaging.md doc still lists setup.py as a minimally necessary doc rather than pyproject.toml. This needs to be updated.
https://github.com/Azure/azure-sdk-for-python/blob/52318772e5a39a150fb55dab5578b01d3f9e5396/doc/dev/packaging.md#what-do-i-do-in-my-files-to-achieve-that