Run httpbin locally instead of using httpbin.org

[janbuchar]

(it's only used in tests)

[janbuchar]

I laid some groundwork for this in #168. Theoretically, all that remains is to actually run httpbin in CI and set the HTTPBIN_URL environment variable accordingly. But.

• it seems we cannot use container services in github actions with the windows runner
• we also cannot install httpbin as a dev dependency, because there is a version requirement conflict that we cannot resolve without dropping support for python <3.12
• the current CI setup runs tests on pull_request_target, which prevents us from testing changes to the CI comfortably

There is a chance that running the httpbin container via docker run -d "directly", like in #167, will work.