Turn some old-style classes into new-style on Py2

* There have been intermittent test failures on travis-ci.org that are very
  hard to reproduce and don't make much sense. We'll see if this helps.
  They look like this:

ERROR: test_attr_entity_replacement (future.tests.test_htmlparser.AttributesStrictTestCase)

----------------------------------------------------------------------

Traceback (most recent call last):
  File "/home/travis/build/edschofield/python-future/future/tests/test_htmlparser.py", line 661, in test_attr_entity_replacement
    [("starttag", "a", [("b", "&><\"'")])])
  File "/home/travis/build/edschofield/python-future/future/tests/test_htmlparser.py", line 93, in _run_check
    collector = self.get_collector()
  File "/home/travis/build/edschofield/python-future/future/tests/test_htmlparser.py", line 617, in get_collector
    return EventCollector(strict=True)
  File "/home/travis/build/edschofield/python-future/future/tests/test_htmlparser.py", line 27, in __init__
    html.parser.HTMLParser.__init__(self, *args, **kw)
  File "/home/travis/build/edschofield/python-future/future/backports/html/parser.py", line 135, in __init__
    self.reset()
  File "/home/travis/build/edschofield/python-future/future/backports/html/parser.py", line 143, in reset
    _markupbase.ParserBase.reset(self)

TypeError: unbound method reset() must be called with ParserBase instance as first argument (got EventCollector instance instead)