www/py-httpx-oauth: add port: Async OAuth client using HTTPX

A generic OAuth2 class is provided to adapt to any OAuth2-compliant service.
Utilities are provided to ease the integration of an OAuth2 process in FastAPI.

WWW: https://github.com/frankie567/httpx-oauth

PR: 260880
Reported by: Goran Mekić <meka@tilda.center> (new maintainer)