Completely move to python 3, updating some libraries

.travis.yml

helios/__init__.py

helios/crypto/algs.py

helios/crypto/electionalgs.py

helios/crypto/elgamal.py

helios/crypto/numtheory.py

helios/crypto/utils.py

helios/datatypes/__init__.py

helios/datatypes/djangofield.py

helios/datetimewidget.py

helios/fields.py

helios/forms.py

helios/management/commands/load_voter_files.py

helios/migrations/0001_initial.py

helios/migrations/0002_castvote_cast_ip.py

helios/migrations/0003_auto_20160507_1948.py

helios/migrations/0004_auto_20170528_2025.py

helios/models.py

helios/security.py

helios/stats_views.py