The test suite segfaults under PyPy 5.9.0, in particular when using multiple inheritance of both a Python and C++ class with the Python class first. It seems in such a case our custom tp_new (in pybind11_object) doesn't get called, so the instance doesn't get allocated or initialized.
I'm creating this issue to document/track it; more details are at the upstream bug report: https://bitbucket.org/pypy/pypy/issues/2674/cpyext-tp_new-not-called-in-some-multiple