python3-h11 (little HTTP/1.1 library written from scratch in Python)

It's a "bring-your-own-I/O" library; h11 contains no IO code
whatsoever. This means you can hook h11 up to your favorite network
API, and that could be anything you want: synchronous, threaded,
asynchronous, or your own implementation of RFC 6214 - h11 won't judge
you.