Do the necessary changes so that the main module can be imported inside
a Python 3 project without errors.
Note that the project will still not work in Python 3, but a project
that imports NEO unconditionally and does not use it in Python 3, such
as wendelin.core, won't have problems.

The changes here are mostly:
  - Replacing old-style excepts (`except Exception, e:`) with the new
  syntax (`except Exception as e:`), compatible with both Python 2 and
  3.
  - Removing calls to `@apply` and replacing them by an explicit call
  to the class/method.
  - Using `six` for some modules that were renamed (e.g. `thread` is
  now `_thread`), as well as some renamed methods `iteritems` is now
  `items`.
  - Replacing `raise` with a tuple of two elements with the appropriate
  call to the exception constructor.
  - Using `six` for achieving a `reraise` with an existing traceback,
  as the old system (raising a tuple of three elements) cannot be used
  in Python 3.
  - I also changed the `requires_extra` for `tests` to include all other
  extras as intended, because the way it was being done did not work
  for me.

The changes are intended to be as surgical and non-controversial as
possible, in order to facilitate review and acceptance.
I can try to achieve full compatibility with Python 3 in future MRs, if
that is desired.
I ran the unittests locally in Python 2 and they seemed to work.