Add type hints to warn in deprecations.py

The `warn` method id `deprecations.py` wasn't typed. Because of this,
calling to the method elsewhere would raise typing errors of calling
to an "untyped method"

core/dbt/deprecations.py

@@ -138,7 +138,7 @@ def cb():
     return cb
 
 
-def warn(name, *args, **kwargs):
+def warn(name: str, *args, **kwargs) -> None:
     if name not in deprecations:
         # this should (hopefully) never happen
         raise RuntimeError("Error showing deprecation warning: {}".format(name))