v0.19.4 - Hot-fix: do not fail if pydantic install is corrupted

On Python3.8, it is fairly easy to get a corrupted install of pydantic (more specificially, pydantic 2.x cannot run if tensorflow is installed because of an incompatible requirement on typing_extensions). Since pydantic is an optional dependency of huggingface_hub, we do not want to crash at huggingface_hub import time if pydantic install is corrupted. However this was the case because of how imports are made in huggingface_hub. This hot-fix releases fixes this bug. If pydantic is not correctly installed, we only raise a warning and continue as if it was not installed at all.

Related PR: #1829

Full Changelog: v0.19.3...v0.19.4