setup: make pimport a setuptools script

[drewrisinger]

The pimport script seems redundant if using the setuptools-based console scripts: https://setuptools.readthedocs.io/en/latest/userguide/entry_point.html#console-scripts

This PR changes pass-import to use that convention.

I'm open to suggestions as to what I'm missing about pimport's usage, but it seems to me that the entry_points command is better-supported than scripts, and it saves having a redundant file.

In a specific use-case, on NixOS/nixpkgs, the scripts option is actually NOT desired, because the pimport script is simply copied and then needs to be manually wrapped to recognize the python environment, instead of using the built-in infrastructure for handling entry_points. See NixOS/nixpkgs#117165.

[roddhjav]

Thanks, merged.