httpbin test dependency is unmaintained

[twm]

httpbin, which is used by treq's test suite (but not elsewhere), hasn't seen a release since 2018. The ecosystem of libraries it relies on is accumulating changes. Without maintenance it will eventually break.

This first became a problem in #352, which relates to deprecated Werkzeug symbols. #353 pinned httpbin and werkzeug (which also fixed compatibility with markupsafe).

Eventually we will have to vendor or replace httpbin. At that point we should also reconsider having so many integration tests — they are slow relative to the coverage they provide.