Add ability to generate `nbdev_pypi` output without uploading to PyPi

[rossheat]

Uploading the dist directory to PyPi in response to a new release using GH Actions is a common workflow.

Hence, having the ability to generate the dist directory without uploading it to PyPI would be useful.

While nbdev_pypi both creates and uploads, perhaps we could add a nbdev_dist command to address this?

[jph00]

Or maybe a "--no_upload" flag?

Would you be interested in trying a PR to add that? (No worries if not - I can do it otherwise)