This is great: I've never used treq, but I don't seen any problem including a new generator, and it's good that you added tests for it.

Two questions:

1. Do you want to add a treq build to the examples, something like writeTypedPythonForAPI testApi treq (result </> "api_treq.py") at the bottom of this module?

2. I never throw blank Exceptions in Python. Do you want to create a custom exception or throw something closer to the type of Exception we're expecting to see here. (Also, does this Exception line work: does it coerce the resp into a string?)