monkeypatch class bases fails with TypeError

In one of our test suites, I'm trying to patch a class' __bases__ (to demonstrate alternate but unexposed behavior), but when I do, monkeypatch fails on the teardown stage with a TypeError:

$ cat test-monkeypatch.py
import pytest


class Loud:
    def thing(self):
        return '!!!!'

class Quiet:
    def thing(self):
        return 'sssh...'


class ThingToTest(Loud):
    pass


class TestThing:
    @pytest.fixture(autouse=True)
    def make_thing_quiet(self, monkeypatch):
        new_bases = (
            Quiet,
        ) + ThingToTest.__bases__[1:]
        monkeypatch.setattr(ThingToTest, '__bases__', new_bases)

    def test_thing_is_quiet(self):
        assert ThingToTest().thing() == 'sssh...'
$ python -m rwt pytest -- -m pytest test-monkeypatch.py
Loading requirements using pytest
====================================== test session starts =======================================
platform darwin -- Python 3.6.0b1, pytest-3.0.2, py-1.4.31, pluggy-0.3.1
rootdir: /Users/jaraco/Dropbox/code/yg/gryphon.cases, inifile: pytest.ini
collected 1 items 

test-monkeypatch.py .E

============================================= ERRORS =============================================
_______________________ ERROR at teardown of TestThing.test_thing_is_quiet _______________________

self = <_pytest.monkeypatch.MonkeyPatch object at 0x1039610b8>

    def undo(self):
        """ Undo previous changes.  This call consumes the
            undo stack. Calling it a second time has no effect unless
            you do more monkeypatching after the undo call.

            There is generally no need to call `undo()`, since it is
            called automatically during tear-down.

            Note that the same `monkeypatch` fixture is used across a
            single test function invocation. If `monkeypatch` is used both by
            the test function itself and one of the test fixtures,
            calling `undo()` will undo all of the changes made in
            both functions.
            """
        for obj, name, value in reversed(self._setattr):
            if value is not notset:
                setattr(obj, name, value)
            else:
>               delattr(obj, name)
E               TypeError: can't delete ThingToTest.__bases__

/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/rwt-vduaqgcq/_pytest/monkeypatch.py:241: TypeError
=============================== 1 passed, 1 error in 0.08 seconds ================================

The same error happens on Python 3.5.2.

I have to monkeypatch the class' __bases__ because the target class (ThingToTest in this example) is already the base for other classes, and I want to avoid reconstructing all of those (plus the fixture which is already an instance of the class).

What's odd is that I'm not setting raises to True, so I would expect it to raise an exception if the class didn't have the __bases__ attribute. And when I invoke getattr or hasattr on the class, it does have the expected attribute.

I'll dig into the monkeypatch implementation to see if I can discover the cause.

Reproduce the failure with test-monkeypatch.py, then inspect pytest's monkeypatch implementation around the undo method and the setattr handling shown at monkeypatch.py line 241. Check how bases is recorded and restored; done means the test still passes and fixture teardown completes without the TypeError.