Using OAuth for a simple command line script to access Google's data

I needed to write a simple script to pull some data from a Google website. Since I was grabbing some private data, I needed authorize myself to do that. I found it much more work than I expected, not because it's hard, but because there wasn't much documentation there to guide me - I had to puzzle out what path to go based on lots of not particularly relevant documentation. So once I'd figured it out I decided to write a short account of what I'd done, partly in case I need to do this again, and partly to help anyone else who wants to do this.I first did this back in 2015. A year or so later, it broke, and I didn't have the bandwidth to fix it. I finally did fix it in 2019. While the libraries I'd used had changed (for the better), the documentation was still rather lacking, so I updated this article.