integration tests pytest fixture for running local http server

[jonemo]

Currently not asking for a thorough review. I only want second (and third and fourth) opinion on whether to keep working on this or to cut my losses and scrap it.

Currently, we make requests to public websites like https://aws.amazon.com as part of our integration tests. While writing integration tests for HTTP client implementations, I figured: Why not instead run a server locally as a pytest fixture? Seems easy enough with Python's http.server.SimpleHTTPRequestHandler. Several rabbit hole explorations later I suddenly find myself passing ctyped locks into processes managed with multiprocessing and working around bugs in type annotations of the more obscure corners of Python's stdlib.

Benefits of having a local server in a fixture:

1. Faster tests because no external network requests
2. Tests can run without access to internet
3. Tests no longer depend on websites that could change any day
4. We can control the HTTP response, for example by reflecting request content
5. Full control over details like HTTP version and header values (caveat: SimpleHTTPRequestHandler has limited support for features of HTTP, for example, no HTTP2)

Downsides of having a local server in a fixture:

1. Spinning the server up and down takes time. (Mitigation: Scoping the fixture at module level amortizes this over many tests.)
2. If for whatever reason the process and/or thread containing the server doesn't shut down, the test run stalls in difficult to debug ways.

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