chore: use weakref.proxy to avoid circular referencing

ibis/backends/__init__.py

@@ -121,7 +121,7 @@ def _ipython_key_completions_(self) -> list[str]:
 class DDLAccessor:
     """ddl accessor list views."""
 
-    def __init__(self, backend: BaseBackend):
+    def __init__(self, backend: weakref.proxy):
         self._backend = backend
 
     def _raise_if_not_implemented(self, method_name: str):
@@ -1053,7 +1053,7 @@ def tables(self):
     @functools.cached_property
     def ddl(self):
         """A ddl accessor."""
-        return DDLAccessor(self)
+        return DDLAccessor(weakref.proxy(self))
 
     @property
     @abc.abstractmethod