add column.n_unique (#337)

Author: MarcoGorelli

spec/API_specification/dataframe_api/column_object.py

@@ -750,6 +750,17 @@ def var(
         """
         ...
 
+    def n_unique(self, *, skip_nulls: bool = True) -> Scalar:
+        """Return number of unique values.
+
+        Notes
+        -----
+        If the original column(s) contain multiple `'NaN'` values, then
+        they only count as one distinct value.
+        Likewise for null values (if ``skip_nulls=False``).
+        """
+        ...
+
     def cumulative_max(self) -> Self:
         """Reduction returns a Column.