Ignore pytools warning in CI

The newest release of pytools (used by pyopencl which is used by silx/pyFAI) made one of the hashing lib siphash24 optional (https://github.com/inducer/pytools/pull/250).

It then now raises a warning if siphash24 is not installed:

pytools.persistent_dict: unable to import 'siphash24.siphash13', falling back to hashlib.sha256

Honestly, I don't think we care much which hashing is used in a sub-dep of a sub-dep so I simply ignored the warning instead of proposing a fix upstream in silx or pyFAI.