make partialmethod example work (closes #21105)

Doc/library/functools.rst

@@ -218,6 +218,8 @@ The :mod:`functools` module defines the following functions:
    Example::
 
       >>> class Cell(object):
+      ...     def __init__(self):
+      ...         self._alive = False
       ...     @property
       ...     def alive(self):
       ...         return self._alive