Investigate use of unicode_literals in the 3.x branch.

A few modules in the 3.0-dev branch of PyMongo import unicode_literals from __future__. This is causing minor problems in the auth module due to hashlib.pbkdf2_hmac expecting a 'str' as its first argument. There may be issues in other modules as well.

Before releasing 3.0 GA we must audit our use of unicode_literals. It may make more sense to remove the __future__ import and work around any issues manually.

The modules to audit are auth.py, bulk.py, and collection.py.