Bump httpx to 0.27.0

[dcermak]

Summary

httpx version 0.27.0 deprecated the app= shortcut, which makes uvicorn's testsuite now fail in openSUSE. This PR fixes the tests for httpx 0.27.0 by relying on ASGITransport(app=app) instead.

Checklist

• [x] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [x] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [x] I've updated the documentation accordingly.

[dcermak]

Type checking currently fails due to https://github.com/encode/httpx/pull/3109 not being in the httpx 0.27 release

[dcermak]

You can add # type: ignore on those.

Done. Should I add FIXMEs to remove this once the next httpx release is out?

[Kludex]

You can add # type: ignore on those.

Done. Should I add FIXMEs to remove this once the next httpx release is out?

No. We already error on unused type: ignores.

[Kludex]

@dcermak Thanks! 🙏