3.6.0 can't find fixtures for cheroot.test.webtest.WebCase based test classes

[lsmithso]

A test class derived from cheroot.test.webtest.WebCase can no longer
find fixtures with pytest 3.6.0. It used to work in 3.5.1.

In the following example, both test cases fail in 3.5.1 because both
test class fixtures are foumd. In 3.6.0 only the test case derived
from object fails because the test case derived from
cheroot.test.webtest.WebCase is not found.

import pytest
from cheroot.test import webtest

class Test1(webtest.WebCase):

@pytest.fixture
def fixture(self):
    assert 0

@pytest.mark.usefixtures('fixture')
def test_cheroot_base(self):
    pass

class Test2(object):

@pytest.fixture
def fixture(self):
    assert 0

@pytest.mark.usefixtures('fixture')
def test_object_base(self):
    pass

python --version
Python 2.7.12

pip list
Package Version

---

atomicwrites 1.1.5
attrs 18.1.0
backports.functools-lru-cache 1.5
cheroot 6.3.1
funcsigs 1.0.2
more-itertools 4.2.0
pip 10.0.1
pluggy 0.6.0
py 1.5.3
pytest 3.6.0
setuptools 39.2.0
six 1.11.0
wheel 0.31.1

[pytestbot]

GitMate.io thinks possibly related issues are #3039 (Fixture not found in test, but found in list of fixtures), #2618 (Propagating fixtures to test class attribute), #386 (tests fail with pexpect-3.0), #2959 (pytest 3.3.0 immutable fixture params), and #3503 (3.6.0 breaking test: internal error loading conftest.py).

[RonnyPfannschmidt]

this was fixed in #3500 and is a duplicate of #3498