TYP: change to FrameOrSeriesUnion

Author: ivanovmg

pandas/core/base.py

@@ -154,14 +154,8 @@ class SelectionMixin:
     Sub-classes need to define: obj, exclusions
     """
 
-    obj: Any  # TODO: refine this type, should NOT be Any
-    """Target object for the selection and aggregation."""
-    # GH 38239
-    # TODO obj here must be typed as FrameOrSeriesUnion,
-    # however this creates multiple mypy errors elsewhere.
-    # Those have to be addressed in a separate PR.
+    obj: FrameOrSeriesUnion
     exclusions: Set[Hashable]
-    """Columns to exclude."""
 
     _selection: Optional[IndexLabel] = None
     _internal_names = ["_cache", "__setstate__"]