Document that dict.get takes no keyword arguments

Running

```python
{}.get("a", default="b")
```

gives:

```
    {}.get("a", default="b")
    ~~~~~~^^^^^^^^^^^^^^^^^^
TypeError: dict.get() takes no keyword arguments
```

Doc/library/stdtypes.rst

@@ -4640,7 +4640,7 @@ can be used interchangeably to index the same dictionary entry.
       such as an empty list.  To get distinct values, use a :ref:`dict
       comprehension <dict>` instead.
 
-   .. method:: get(key, default=None)
+   .. method:: get(key, default=None, /)
 
       Return the value for *key* if *key* is in the dictionary, else *default*.
       If *default* is not given, it defaults to ``None``, so that this method