collections.UserString.format and collections.UserString.format_map return a string instaed of UserString.

This is quite weird because I expect that the %-syntax and `format` method have to produce the same result.

```
>>> isinstance(UserString('Hello %s') % 'World', UserString)
True
>>> isinstance(UserString('Hello {}').format('World'), UserString)
False
```

It is too late is the life of UserString for a design change, so I'm going to have to close this as more likely to harm existing code than to benefit future code.

Also, despite you expectations, this isn't atypical for Python classes.  For example, the __add__ method on integer subclasses returns actual ints rather than instances of the subclass.

In the case of UserString, the current design can be useful because it make UserString instances more substitutable for regular strings.