Fix code-block in zoneinfo (GH-20201)

```
Warning, treated as error:
/tmp/code/Doc/library/zoneinfo.rst:303:Error in "code-block" directive:
1 argument(s) required, 0 supplied.

.. code-block::

    >>> a = ZoneInfo("Europe/Berlin")
    >>> b = pickle.loads(europe_berlin_pkl)
    >>> a is b
    True
```

Author: asottile

Doc/library/zoneinfo.rst

@@ -300,7 +300,7 @@ The behavior of a ``ZoneInfo`` file depends on how it was constructed:
    constructed from ``ZoneInfo("Europe/Berlin")``, one would expect the
    following behavior:
 
-   .. code-block::
+   .. code-block:: pycon
 
        >>> a = ZoneInfo("Europe/Berlin")
        >>> b = pickle.loads(europe_berlin_pkl)
@@ -314,7 +314,7 @@ The behavior of a ``ZoneInfo`` file depends on how it was constructed:
    constructed from ``ZoneInfo.no_cache("Europe/Berlin")``, one would expect
    the following behavior:
 
-   .. code-block::
+   .. code-block:: pycon
 
        >>> a = ZoneInfo("Europe/Berlin")
        >>> b = pickle.loads(europe_berlin_pkl_nc)