fix: add ExperimentalFeatureWarning compat shim
Existing models import ExperimentalFeatureWarning to suppress warnings:
from cog import ExperimentalFeatureWarning
warnings.filterwarnings('ignore', category=ExperimentalFeatureWarning)
This class was removed when the Python HTTP server was replaced by
coglet, causing ImportError on model load.
The shim provides _ExperimentalFeatureWarning (a FutureWarning subclass)
via module-level __getattr__, which:
- Prints a deprecation notice to stderr (cannot be swallowed by
warnings.filterwarnings('ignore'))
- Caches in globals() so the message fires at most once
- Returns the class so existing filterwarnings() calls still work
