Typing: Narrow down types of argument keep for nsmallest and nlargest (#52753)

* Typing: Narrow down types of argument keep for nsmallest and nlargest

* fixed formatting with black

Author: mattkeanny

pandas/core/series.py

@@ -3901,7 +3901,9 @@ def nlargest(
         """
         return selectn.SelectNSeries(self, n=n, keep=keep).nlargest()
 
-    def nsmallest(self, n: int = 5, keep: str = "first") -> Series:
+    def nsmallest(
+        self, n: int = 5, keep: Literal["first", "last", "all"] = "first"
+    ) -> Series:
         """
         Return the smallest `n` elements.