Use the :exc: Sphinx role for DeprecationWarning

Python warnings are exceptions, to facilitate converting them to
errors by raising them. Thus the more specific :exc: role applies
and is a better fit than the more general :class: role.

git/util.py

@@ -1285,7 +1285,7 @@ def list_items(cls, repo: "Repo", *args: Any, **kwargs: Any) -> IterableList[T_I
 
 
 class IterableClassWatcher(type):
-    """Metaclass that issues :class:`DeprecationWarning` when :class:`git.util.Iterable`
+    """Metaclass that issues :exc:`DeprecationWarning` when :class:`git.util.Iterable`
     is subclassed."""
 
     def __init__(cls, name: str, bases: Tuple, clsdict: Dict) -> None: