Switch CI to treat warnings as errors

[tomchristie]

Prompted by #1670

Since merging #1682 we're now including test runs for Python 3.10 beta.
Issue #1670 notes that we've got a deprecation warning being raised on 3.10.
The sensible approach in terms of process here is to make sure that our CI is erroring on deprecation warnings, so that we're seeing these cases up front, and able able to validate that we've resolved them, and know if we ever unintentionally introduce a regression.

So first step here prior to considering #1670 should be to upgrade our warnings to errors.

That'll also highlight that we've got an existing case on 3.8 and 3.9 of a test cases that's causing a warning that ought to be resolve. (See https://github.com/encode/httpx/pull/1682/checks?check_run_id=2828639691)