Update to Python 3.5 for Bandersnatch 2.0 support

[billmoritz]

Issue: The currently published Docker container (https://hub.docker.com/r/janlo/pypi-mirror-bandersnatch/) doesn't start

Traceback (most recent call last):
  File "/home/user/bandersnatch-env/bin/bandersnatch", line 11, in <module>
    load_entry_point('bandersnatch==2.0.0', 'console_scripts', 'bandersnatch')()
  File "/home/user/bandersnatch-env/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 561, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/home/user/bandersnatch-env/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2631, in load_entry_point
    return ep.load()
  File "/home/user/bandersnatch-env/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2291, in load
    return self.resolve()
  File "/home/user/bandersnatch-env/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2297, in resolve
    module = __import__(self.module_name, fromlist=['__name__'], level=0)
  File "/home/user/bandersnatch-env/local/lib/python2.7/site-packages/bandersnatch/main.py", line 4, in <module>
    import bandersnatch.mirror
  File "/home/user/bandersnatch-env/local/lib/python2.7/site-packages/bandersnatch/mirror.py", line 1, in <module>
    from .package import Package
  File "/home/user/bandersnatch-env/local/lib/python2.7/site-packages/bandersnatch/package.py", line 4, in <module>
    from urllib.parse import urlparse, unquote
ImportError: No module named parse

Cause: https://pypi.python.org/pypi/bandersnatch#id1

2.0.0 (2017-04-05)

Official support starts with Python 3.5 but might get away with using an earlier version of Python 3 (maybe 3.3 or so). However, we plan to start using Python 3.5 features (like asyncio) in the near future, so please be advised that running with an older version of Python 3 is not a supported option for the long term.