Fix SubplotSpec.get_gridspec type hint

SubplotSpec.__init__ accepts an instance of GridSpecBase,
the getter function should return a GridSpecBase as well.

Author: has2k1

lib/matplotlib/gridspec.pyi

@@ -115,7 +115,7 @@ class SubplotSpec:
     def num2(self) -> int: ...
     @num2.setter
     def num2(self, value: int) -> None: ...
-    def get_gridspec(self) -> GridSpec: ...
+    def get_gridspec(self) -> GridSpecBase: ...
     def get_geometry(self) -> tuple[int, int, int, int]: ...
     @property
     def rowspan(self) -> range: ...