Replace `httpx` in the examples and tests

Let's find a replacement for httpx for our frontpage example and a couple of ASGI tests were we use it.

Most popular choices from what I could gather quickly:

• httpx2 -- maintained by @Kludex & co -- probably the most direct replacement / fork?
• niquests -- another requests workalike, looks fairly decent and popular too, although I have never tested it myself.
• aiohttp -- a very well known and proven asynchronous client and server framework, however, not as straightforward to work with, at least not for requests users.