Fix httpx deprecation warnings #2307
Description
Lots of these: https://github.com/simonw/datasette/actions/runs/8258529260/job/22590924898
/opt/hostedtoolcache/Python/3.12.2/x64/lib/python3.12/site-packages/httpx/_client.py:1426:
DeprecationWarning: The 'app' shortcut is now deprecated. Use the explicit style 'transport=ASGITransport(app=...)' instead.
And:
/opt/hostedtoolcache/Python/3.12.2/x64/lib/python3.12/site-packages/httpx/_client.py:1559:
DeprecationWarning: Setting per-request cookies=<...> is being deprecated, because the expected behaviour on cookie persistence is ambiguous. Set cookies directly on the client instance instead.