Add a wrapper not to break pytest 4 (needed for Python 2 support).

============================= test session starts ==============================
platform linux -- Python 3.9.0b5, pytest-6.0.1, py-1.9.0, pluggy-0.13.1
rootdir: /builddir/build/BUILD/html5lib-1.1, configfile: pytest.ini
plugins: expect-1.1.0
collected 0 items / 1 error

==================================== ERRORS ====================================
________________________ ERROR collecting test session _________________________
/usr/lib/python3.9/site-packages/pluggy/hooks.py:286: in __call__
    return self._hookexec(self, self.get_hookimpls(), kwargs)
/usr/lib/python3.9/site-packages/pluggy/manager.py:93: in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
/usr/lib/python3.9/site-packages/pluggy/manager.py:84: in <lambda>
    self._inner_hookexec = lambda hook, methods, kwargs: hook.multicall(
html5lib/tests/conftest.py:105: in pytest_collect_file
    return TokenizerFile(path, parent)
/usr/lib/python3.9/site-packages/_pytest/nodes.py:95: in __call__
    warnings.warn(NODE_USE_FROM_PARENT.format(name=self.__name__), stacklevel=2)
E   pytest.PytestDeprecationWarning: Direct construction of TokenizerFile has been deprecated, please use TokenizerFile.from_parent.
E   See https://docs.pytest.org/en/stable/deprecations.html#node-construction-changed-to-node-from-parent for more details.

Fixes #505