REF: SingleBlockManager dont subclass BlockManager (#40625)

Author: jbrockmendel

pandas/_typing.py

@@ -185,7 +185,7 @@
 ]
 
 # internals
-Manager = Union["ArrayManager", "BlockManager"]
+Manager = Union["ArrayManager", "BlockManager", "SingleBlockManager"]
 SingleManager = Union["SingleArrayManager", "SingleBlockManager"]
 
 # indexing

pandas/core/frame.py

@@ -548,6 +548,7 @@ class DataFrame(NDFrame, OpsMixin):
     _HANDLED_TYPES = (Series, Index, ExtensionArray, np.ndarray)
     _accessors: set[str] = {"sparse"}
     _hidden_attrs: frozenset[str] = NDFrame._hidden_attrs | frozenset([])
+    _mgr: BlockManager | ArrayManager
 
     @property
     def _constructor(self) -> type[DataFrame]: