fix mypy: add new_index parameter to NDFrame._slice base method

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jbrockmendel

pandas/core/generic.py

@@ -4334,7 +4334,9 @@ def _getitem_slice(self, key: slice) -> Self:
             slobj = indexer
         return self._slice(slobj)
 
-    def _slice(self, slobj: slice, axis: AxisInt = 0) -> Self:
+    def _slice(
+        self, slobj: slice, axis: AxisInt = 0, new_index: Index | None = None
+    ) -> Self:
         """
         Construct a slice of this container.