helpers is not compatible with python 3

[Curtis Hovey]

Since Ubuntu raring, python2 is not installed by default. Trusty will be the first LTS to not ship with py2. Charms that use charm-helpers must install python2 to work. It may be possible to rewrite charm-helpers by py2 and py3 compatible so that charms can be deployed on precise and trusty.

$ python3 /usr/bin/pocketlint -q `find . -name '*.py'`
./charmhelpers/contrib/saltstack/__init__.py
     130: undefined name 'unicode'
./charmhelpers/contrib/jujugui/utils.py
     428: Could not compile; invalid syntax: except OSError, e:
./charmhelpers/contrib/openstack/context.py
     274: undefined name 'basestring'
./charmhelpers/fetch/__init__.py
      67: undefined name 'basestring'
      67: undefined name 'basestring'
./charmhelpers/fetch/archiveurl.py
      40: Could not compile; invalid token: mkdir(dest_dir, perms=0755)
./charmhelpers/core/host.py
     110: Could not compile; invalid token: def mkdir(path, owner='root', group='root', perms=0555, force=False):
./tools/charm_helpers_sync/charm_helpers_sync.py
     163: Could not compile; invalid syntax: except Exception, e:
./tests/contrib/network/test_ovs.py
      15: undefined name 'file'
./tests/contrib/jujugui/test_utils.py
       7: 'dumps' imported but unused
./tests/contrib/openstack/test_openstack_utils.py
     295: undefined name 'file'
     330: undefined name 'file'
     382: undefined name 'file'
./tests/contrib/openstack/test_os_contexts.py
      18: undefined name 'file'
./tests/fetch/test_fetch.py
      20: undefined name 'file'
./tests/core/test_host.py
     310: Could not compile; invalid token: perms = 0644
./tests/core/test_hookenv.py
     156: undefined name 'basestring'