Fix type hint for SubplotSpec init

It is only ever created in `GridSpecBase.__getitem__` with `self` as
argument, so must accept those instead of `GridSpec`.

Author: QuLogic

lib/matplotlib/gridspec.pyi

@@ -34,7 +34,9 @@ class GridSpecBase:
     ) -> tuple[np.ndarray, np.ndarray, np.ndarray, np.ndarray]: ...
     @staticmethod
     def _check_gridspec_exists(figure, nrows, ncols): ...
-    def __getitem__(self, key: tuple[int | slice, int | slice] | slice | int) -> SubplotSpec: ...
+    def __getitem__(
+        self, key: tuple[int | slice, int | slice] | slice | int
+    ) -> SubplotSpec: ...
     @overload
     def subplots(
         self,
@@ -105,7 +107,7 @@ class GridSpecFromSubplotSpec(GridSpecBase):
 class SubplotSpec:
     num1: int
     def __init__(
-        self, gridspec: GridSpec, num1: int, num2: int | None = ...
+        self, gridspec: GridSpecBase, num1: int, num2: int | None = ...
     ) -> None: ...
     @staticmethod
     def _from_subplot_args(figure, args): ...