Add missing comma in single-element tuple.

Failing test case:
```python
from pretty import pprint
pprint(object())
```

results in:
```
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "$PREFIX/lib/python3.2/site-packages/pretty.py", line 106, in pprint
    printer.pretty(obj)
  File "$PREFIX/lib/python3.2/site-packages/pretty.py", line 299, in pretty
    return _default_pprint(obj, self, cycle)
  File "$PREFIX/lib/python3.2/site-packages/pretty.py", line 397, in _default_pprint
    if getattr(klass, '__repr__', None) not in _baseclass_reprs:
TypeError: argument of type 'wrapper_descriptor' is not iterable
```