Add support for Python 3.5

[felixonmars]

This addresses #30

[jamadden]

Would it be better to avoid monkey-patching a stdlib module, and instead make _iter_chain a local function? Something like:

try:
    _iter_chain = traceback._iter_chain
except AttributeError:
    # Python 3.5
    def _iter_chain(...):

...
def format_exception(...):
  if chain:
       values = _iter_chain(v, tb)

[felixonmars]

I see, updated :)

[mgedmin]

This looks very similar to traceback._iter_chain() from Python 3.4, but differs in some small details. Where did it come from?

[felixonmars]

Looks like I looked up a somewhat outdated copy of that file, sorry for this. Link to the copy I used: http://svn.python.org/projects/python/branches/pep-0384/Lib/traceback.py

I'm going to update the PR using a more recent version (with a little adjustment to the "yield from" part, as that's an invalid syntax for Python 2)

[mgedmin]

LGTM.

Do we need a new zope.testrunner release on PyPI to support Python 3.5 for other zope.* packages that use zope.testrunner to run their tests?

[felixonmars]

I didn't run into problems with testrunner when running tests of other zope.* packages yet, but a new release would be safer :)