Python2 to Python3 conversion using 2to3

This enables pyicloud to be used with Python3.
Thanks for your work!
This is a little nit-picky, but [per PEP-8, third-party imports should be in their own block](http://legacy.python.org/dev/peps/pep-0008/#imports), separated from standard library imports.  Could you perhaps add a single newline above your import of `pytz`?