Dependence on httpx results in deprecation warnings

Authlib has [recently dropped httpx in favour of httpx2](https://github.com/authlib/authlib/issues/904), resulting in complaints about deprecation warnings on every invocation.

If you're going to depend on authlib, it doesn't make sense to also depend on `httpx` for SchemaRegistry support as it commits your users to being spammed with this warning on every invocation.

As a work-around, users can install httpx2 *as well*, though this isn't ideal if the project doesn't use it directly.

Start by locating the SchemaRegistry support and its direct httpx dependency, then reproduce the deprecation warning during a normal invocation. Done means the supported dependency arrangement no longer produces the reported warning while SchemaRegistry support continues to work.

Written by the indexing model from the issue text.