Support pytest 4

[FRidh]

I suppose the test suite is not pytest 4.2 compatible.

============================= test session starts ==============================
platform linux -- Python 3.7.2, pytest-4.2.1, py-1.7.0, pluggy-0.8.1
rootdir: /build/html5lib-1.0.1, inifile: pytest.ini
plugins: expect-1.1.0
collected 31264 items / 1 errors / 31263 selected

==================================== ERRORS ====================================
________________ ERROR collecting html5lib/tests/test_stream.py ________________
/nix/store/mljhw8f6ng9ny9ahx68m9fj6bq0ghbqv-python3.7-pluggy-0.8.1/lib/python3.7/site-packages/pluggy/hooks.py:284: in __call__
    return self._hookexec(self, self.get_hookimpls(), kwargs)
/nix/store/mljhw8f6ng9ny9ahx68m9fj6bq0ghbqv-python3.7-pluggy-0.8.1/lib/python3.7/site-packages/pluggy/manager.py:68: in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
/nix/store/mljhw8f6ng9ny9ahx68m9fj6bq0ghbqv-python3.7-pluggy-0.8.1/lib/python3.7/site-packages/pluggy/manager.py:62: in <lambda>
    firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
/nix/store/pf8c7km0691nci67allvq2ggsgvr8asl-python3.7-pytest-4.2.1/lib/python3.7/site-packages/_pytest/python.py:226: in pytest_pycollect_makeitem
    res = list(collector._genfunctions(name, obj))
/nix/store/pf8c7km0691nci67allvq2ggsgvr8asl-python3.7-pytest-4.2.1/lib/python3.7/site-packages/_pytest/python.py:403: in _genfunctions
    self.ihook.pytest_generate_tests(metafunc=metafunc)
/nix/store/mljhw8f6ng9ny9ahx68m9fj6bq0ghbqv-python3.7-pluggy-0.8.1/lib/python3.7/site-packages/pluggy/hooks.py:284: in __call__
    return self._hookexec(self, self.get_hookimpls(), kwargs)
/nix/store/mljhw8f6ng9ny9ahx68m9fj6bq0ghbqv-python3.7-pluggy-0.8.1/lib/python3.7/site-packages/pluggy/manager.py:68: in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
/nix/store/mljhw8f6ng9ny9ahx68m9fj6bq0ghbqv-python3.7-pluggy-0.8.1/lib/python3.7/site-packages/pluggy/manager.py:62: in <lambda>
    firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
/nix/store/pf8c7km0691nci67allvq2ggsgvr8asl-python3.7-pytest-4.2.1/lib/python3.7/site-packages/_pytest/python.py:133: in pytest_generate_tests
    metafunc.parametrize(*marker.args, **marker.kwargs)
/nix/store/pf8c7km0691nci67allvq2ggsgvr8asl-python3.7-pytest-4.2.1/lib/python3.7/site-packages/_pytest/python.py:987: in parametrize
    function_definition=self.definition,
/nix/store/pf8c7km0691nci67allvq2ggsgvr8asl-python3.7-pytest-4.2.1/lib/python3.7/site-packages/_pytest/mark/structures.py:114: in _for_parametrize
    if len(param.values) != len(argnames):
E   TypeError: object of type 'MarkDecorator' has no len()

[hroncok]

Some deprecation warnings from before:

=============================== warnings summary ===============================
/usr/lib/python3.7/site-packages/_pytest/python.py:764
  /usr/lib/python3.7/site-packages/_pytest/python.py:764: RemovedInPytest4Warning: usage of Generator.Function is deprecated, please use pytest.Function instead
    function_class = self.Function
html5lib/tests/test_encoding.py:98
  /builddir/build/BUILD/html5lib-1.0.1/html5lib/tests/test_encoding.py:98: RemovedInPytest4Warning: yield tests are deprecated, and scheduled to be removed in pytest 4.0
    def test_encoding():
/usr/lib/python3.7/site-packages/_pytest/python.py:764
  /usr/lib/python3.7/site-packages/_pytest/python.py:764: RemovedInPytest4Warning: usage of Generator.Function is deprecated, please use pytest.Function instead
    function_class = self.Function
html5lib/tests/test_sanitizer.py:62
  /builddir/build/BUILD/html5lib-1.0.1/html5lib/tests/test_sanitizer.py:62: RemovedInPytest4Warning: yield tests are deprecated, and scheduled to be removed in pytest 4.0
    def test_sanitizer():
html5lib/tests/test_serializer.py:220
  /builddir/build/BUILD/html5lib-1.0.1/html5lib/tests/test_serializer.py:220: RemovedInPytest4Warning: yield tests are deprecated, and scheduled to be removed in pytest 4.0
    def test_serializer():
html5lib/tests/test_stream.py:300
  html5lib/tests/test_stream.py:300: RemovedInPytest4Warning: Applying marks directly to parameters is deprecated, please use pytest.param(..., marks=...) instead.
  For more details, see: https://docs.pytest.org/en/latest/parametrize.html
    @pytest.mark.skipif(not supports_lone_surrogates, reason="doesn't support lone surrogates")
html5lib/tests/test_treewalkers.py:81
  /builddir/build/BUILD/html5lib-1.0.1/html5lib/tests/test_treewalkers.py:81: RemovedInPytest4Warning: yield tests are deprecated, and scheduled to be removed in pytest 4.0
    def test_treewalker_six_mix():
-- Docs: https://docs.pytest.org/en/latest/warnings.html

[hroncok]

I'll try to submit a PR.