[HTTPXodus] 考虑从 `httpx` 迁移到 `httpx2` (正在维护的分支)

> ️ This issue is part of HTTPXodus, a community effort to help major projects plan their path off the stalled httpx stable line onto httpx2, the actively maintained fork by Pydantic Services (with original httpx author Tom Christie involved). One coordinated issue per project — no spam, no drive-by PRs.

背景 — httpx 发生了什么事