Update process for uploading to PyPi

[bellisk]

Trying to upload the latest release to PyPi, I got this error, which called to my attention the fact that building and uploading via setup.py have been deprecated for some time now.

It seems we should probably switch to using build and twine for this purpose (see blog post here), but I don't know what else that would affect or whether there is any coordinated effort to update this for ckanext packages in general - so I'm creating this ticket for any needed discussion.