_pprint_dict forces dictionary-style output for objects that inherit from dict
Because pprint_thing checks isinstance(thing, dict), it also catches objects that inherit from dict and forces them to print in the {'a':1, ... } format regardless of the object's __unicode__, __str__, or __repr__ definitions. On my machine, I've changed the _pprint_dict function to just:

```
return unicode(seq)
```

which seems to give the behavior I'm looking for. I see pprint_thing warns against unicode(object) in general due to nested sequences, but since _pprint_dict doesn't do anything with _nest_lvl anyway, I figured it was probably safe. 
