My findings:

The part

self._client_kwargs = {
            "timeout": timeout,
            "proxies": proxy_url,
            "limits": limits,
            "http1": http1,
            "http2": not http1,
        }

in HTTPXRequest is what breaks the CI. Changing to

self._client_kwargs = {}
self._client_kwargs["timeout"] = timeout
...

does not work either, but

self._client_kwargs = dict()
self._client_kwargs["timeout"] = timeout
...

does, so it's definitely the curly braces.

• I still can't reproduce this locally, even on the same python version and with the same packages installed
• It's still not clear to me what exactly the problem even is. The log files read a bit like the recursion depth error happens while a traceback is being formatting, but I don't know which

Since this is hard to debug and it's only this one place that acts up, I'll just go back to the dict(…) approach. Maybe we find a solution in the future …