You can also use Caddy to do this. Single static binary, use the "respond" command to spin up a super simple server that responds with the text and headers you want: https://caddyserver.com/docs/command-line#caddy-respond also gives you Automatic HTTPS out of the box, production ready proxy, etc.

A few years ago I put together: https://github.com/nickjj/webserver

It's a self hosted zero dependency / single file Python based echo server. You run `webserver 8000` and then you can make HTTP requests to `localhost:8000` with whatever URL path, headers and payload you want and it'll echo it back.

Thank you for your comment! I plan to add it in the next implementation.