I don't want to take anything away from requests. It's an extremely useful library. However, I do a code walkthrough of various popular libraries with my students and I've never been satisfied with requests internals. The emphasis, I've felt, is more on clean, stable, public API rather than internal consistency and cleanliness. I don't pick it up these days for my classes. Instead, we usually go through the python standard library. Especially the code from Raymond Hettinger.
We usually do collections. Especially the implementation of the OrderedDict. It's a nice combination of compromises, careful coding and usefulness.
I used to do requests but like I said, moved away. I sometimes do pygments to discuss how different lexers etc. are "plugged" in and also to show how messy the command line parsing code there is.