`httpretty` looks unmaintained

[StevenMaude]

As opensafely-core/reports#1014:

Noticed while running just devenv:

DEPRECATION: Building 'httpretty' using the legacy setup.py bdist_wheel mechanism, which will be removed in a future version.

The actual version we have is the latest (1.1.4), but it is from 2021.

So the package:

• looks no longer maintained
• has quirks that we may be encountering or will encounter, and aren't likely to be fixed

Because of that, it seems sensible to replace it.

[StevenMaude]

Work-in-progress: https://github.com/opensafely-core/osgithub/tree/steve/replace-httpretty-with-responses