rename groupby to group_by (#259)

Author: MarcoGorelli

spec/API_specification/dataframe_api/dataframe_object.py

@@ -65,7 +65,7 @@ def shape(self) -> tuple[int, int]:
         Return number of rows and number of columns.
         """
 
-    def groupby(self, keys: str | list[str], /) -> GroupBy:
+    def group_by(self, keys: str | list[str], /) -> GroupBy:
         """
         Group the DataFrame by the given columns.
 

spec/API_specification/dataframe_api/groupby_object.py

@@ -14,7 +14,7 @@ class GroupBy:
     GroupBy object.
 
     Note that this class is not meant to be constructed by users.
-    It is returned from `DataFrame.groupby`.
+    It is returned from `DataFrame.group_by`.
 
     **Methods**