BUG: Replace isinstance with issubclass in _ensure_type (pandas-dev#31925)

Commit pandas-dev/pandas@6fd326d in
pull request pandas-dev#30613 added `_ensure_type`, which
utilizes `isinstance`. However, it is reasonable to assume that someone
may want to create a DataFrame subclass. Therefore, `_ensure_type`
should use `issubclass`.

Author: ericchansen

pandas/core/base.py

@@ -93,7 +93,7 @@ def _ensure_type(self: T, obj) -> T:
 
         Used by type checkers.
         """
-        assert isinstance(obj, type(self)), type(obj)
+        assert issubclass(type(obj), type(self)), type(obj)
         return obj