builtins.pyi: Return complex from __(r)pow__() (#2662)

Closes #1406

Author: utkarsh2102

stdlib/3/builtins.pyi

@@ -196,7 +196,7 @@ class float:
     def __truediv__(self, x: float) -> float: ...
     def __mod__(self, x: float) -> float: ...
     def __divmod__(self, x: float) -> Tuple[float, float]: ...
-    def __pow__(self, x: float) -> float: ...
+    def __pow__(self, x: float) -> float: ...  # Returns complex if self is negative and x is not whole
     def __radd__(self, x: float) -> float: ...
     def __rsub__(self, x: float) -> float: ...
     def __rmul__(self, x: float) -> float: ...