Flaky test_fetch_redirect depends on 3rd-party server

test_fetch_redirect has been flaking quite a bit lately, and the only reason seems to be because it depends on a 3rd-party server (httpbin.org) to return a redirect header.

It should be pretty straightforward to replace that with a tiny local HTTP server instead, e.g. via [http.server](https://docs.python.org/3/library/http.server.html).