fix for NamedAgg in agg and aggregate (#138)

Author: bschnurr

pandas/core/groupby/generic.pyi

@@ -66,13 +66,15 @@ class DataFrameGroupBy(GroupBy):
     @overload
     def aggregate(self, arg: Dict, *args, **kwargs) -> DataFrame: ...
     @overload
+    def aggregate(self, arg: F | None = ..., *args, **kwargs) -> DataFrame: ...
+    @overload
     def aggregate(self, arg: Callable[[], Any], *args, **kwargs) -> DataFrame: ...
     @overload
     def agg(self, arg: str, *args, **kwargs) -> DataFrame: ...
     @overload
     def agg(self, arg: Dict, *args, **kwargs) -> DataFrame: ...
     @overload
-    def agg(self, arg: F, *args, **kwargs) -> DataFrame: ...
+    def agg(self, arg: F | None = ..., *args, **kwargs) -> DataFrame: ...
     def transform(self, func, *args, **kwargs): ...
     def filter(self, func: Callable, dropna: bool = ..., *args, **kwargs) -> DataFrame: ...
     @overload