Fix float.__rpow__ (#7047)

Fixes #7046.

Author: JelleZijlstra

stdlib/builtins.pyi

@@ -296,7 +296,8 @@ class float:
     def __rtruediv__(self, __x: float) -> float: ...
     def __rmod__(self, __x: float) -> float: ...
     def __rdivmod__(self, __x: float) -> tuple[float, float]: ...
-    def __rpow__(self, __x: float, __mod: None = ...) -> float: ...
+    # Returns complex if the argument is negative.
+    def __rpow__(self, __x: float, __mod: None = ...) -> Any: ...
     def __getnewargs__(self) -> tuple[float]: ...
     def __trunc__(self) -> int: ...
     if sys.version_info >= (3, 9):