Async client: RFC 6764 discovery needs a sync HTTP library #860

Disclaimer: AI-generated text below the line, generated by Claude Opus 5.5, from the prompt(s):

merge #855 and #856
Open a github issue [for the problem found while fixing the CI that blocked those PRs]

caldav/discovery.py imports the sync HTTP library at module level (from caldav.lib.http_sync import requests, line 47) and makes the .well-known lookup with requests.get(). The async client uses it too: get_davclient() in caldav.async_davclient runs RFC 6764 discovery in a worker thread. So an install with only an async library (httpx, httpxyz or httpx2) and neither niquests nor requests raises ImportError: caldav needs an HTTP library for sync communication ... as soon as discovery is reached, although no sync client is in use.

Today it is hidden because niquests is a hard dependency. It matters once that changes (the v4.0 plan in pyproject.toml), and it keeps the CI fallback jobs from testing a truly async-only install: they now install requests explicitly (#857). It surfaced when Radicale 3.9.1 stopped pulling in requests, and test_get_davclient_discovers_off_the_event_loop failed in the async fallback jobs.

Possible fixes:

• import the sync library lazily inside the function that does the HTTP lookup, and let the async path do that lookup with its own library;
• or do the small .well-known GET with the standard library (urllib.request), so discovery needs no HTTP library at all.

Done when an async (httpx2 fallback)-style CI job runs without requests installed and the discovery test passes.