Fix error in Python 3

```python
(.env) ~/D/django-statsd (master|✔) $ nosetests
E
======================================================================
ERROR: Failure: ImportError (No module named unittest2)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/site-packages/nose/loader.py", line 413, in loadTestsFromName
    addr.filename, addr.module)
  File "/usr/local/lib/python2.7/site-packages/nose/importer.py", line 47, in importFromPath
    return self.importFromDir(dir_path, fqname)
  File "/usr/local/lib/python2.7/site-packages/nose/importer.py", line 94, in importFromDir
    mod = load_module(part_fqname, fh, filename, desc)
  File "/Users/coreyf/Development/django-statsd/django_statsd/tests.py", line 8, in <module>
    from unittest2 import skipUnless
ImportError: No module named unittest2

----------------------------------------------------------------------
Ran 1 test in 0.017s

FAILED (errors=1)
```