refactor: pandas-dev#1238 simplify typing

Author: cmp0xff

pandas-stubs/_typing.pyi

@@ -1011,7 +1011,7 @@ TimeZones: TypeAlias = str | tzinfo | None | int
 
 # Evaluates to a DataFrame column in DataFrame.assign context.
 IntoColumn: TypeAlias = (
-    AnyArrayLike | Scalar | Callable[[DataFrame], AnyArrayLike | Scalar]
+    AnyArrayLike | Scalar | Callable[[DataFrame], AnyArrayLike | Scalar] | None
 )
 
 DatetimeLike: TypeAlias = datetime.datetime | np.datetime64 | Timestamp

pandas-stubs/core/frame.pyi

@@ -794,7 +794,7 @@ class DataFrame(NDFrame, OpsMixin, _GetItemHack):
         value: Scalar | ListLikeU | None,
         allow_duplicates: _bool = ...,
     ) -> None: ...
-    def assign(self, **kwargs: IntoColumn | None) -> Self: ...
+    def assign(self, **kwargs: IntoColumn) -> Self: ...
     def align(
         self,
         other: NDFrameT,