Pin httpbin 0.7.0 and werkzeug 2.0.3

[twm]

Fixes #352.

httpbin 0.7.0 gets us compatibility with current versions of MarkupSafe, which dropped soft_unicode in version 2.1.0.

werkzeug 2.0.3 is the most recent version that includes the deprecated BaseResponse symbol that httpbin imports.

This is just a quick fix. To properly address this we'll have to address the fact that httpbin hasn't released since 2018. This also doesn't address the very lax pinning of test dependencies in general.