Httpx intercept

[cdent]

httpx provides a more modern approach to python+http with
a straightforward API that maps cleanly to gabbi/httpclient.py.

It also, in the future, will allow support for http/2.

The change switches out urllib3 from under httpclient,
replacing it with httpx.

It does not address changes required to get interception
of wsgi applications working. httpx provides its own
way of doing interception that is much different from
the old hack of wsgi-intercept. Those changes will be
next.

Replace wsgi-intercept, used in gabbi as a feature
to install the intercept and do http over a monkey-
patched socker.

The replacement is WSGITransport from httpx. This works
in a different way. Instead of monkey-patching, a
different transport is dependency-injected into the
http client. Each gabbi tests has its own client
instance.

Because of this change in concept the diff, though
not especially big, touches several parts of the code.

1. Fixture handling removes use of the wsgi-intecept
   context manager.
2. intercept and prefix are based to httpclient generation.
   intercept signals that the WSGITransport should be
   used.
3. An external server process (in tests/external_server.py)
   is used by tests/test_runner.py so that gabbi-run
   can be tested effectively without wsgi-intercept.
4. Step 3 identified some long present bugs in the
   SimpleWSGI test app to do with reading content
   from POST and PUT operations.
5. Prefix handling and url-generation are different when
   using WSGITransport so simplewsgi has a different way
   of constructing fully qualified URLs. This uses
   removeprefix which is no supported in python 3.8, so
   support for 3.8 is dropped. 3.8 was EOL in 2024.
6. Special tests which had all been depdending on the older
   python 3.9 now use 3.13. This is mostly because that
   made it is easier for cdent to do testing locally, but
   seems good hygiene anyway.
7. pypy3 is upgrade to pypy3.10 as that's what's available
   these days.
8. Docs have tried to be updated but it is likelys something
   has been missed.
9. Some shenanigans with how hostnames are handled in SimpleWSGI
   needed to be reconciled between what a mac does and what a
   github ci node does.

This patch does not try to prepare a release. That will
be later.

[cdent]

Current failures are due to difference with host ip resolution between my Mac (which is doing weird stuff) and the CI runners (which are behaving in what I would think is the normal way).

Will provide details and a real PR in the near future @scottwallacesh , @FND but pinging here in case you are curious about progress. Apologies for the terrible commit messages, that will be fixed once this is "real". I've been...distracted.

[scottwallacesh]

Looks good to me. I can see it's still referencing wsgi-intercept in the requirements.txt and a few other places (host.rst, driver.py and host-headers.yaml) - not sure if they can go or not.

[cdent]

I can see it's still referencing wsgi-intercept

Yeah, there's some cleanups to do, as well as documentation updates before we can call this something like done.