[BUG] def_property_readonly_static calling PyErr_WarnEx fails on PyPy

Issue description

When calling PyErr_WarnEx from inside the lambda given to def_property_static_readonly, it fails with KeyError, but only when run under PyPy.

Perhaps this is a PyPy bug, but I don't really know where I'd start debugging that.

PyPy 7.3.5, pybind11 2.6.2, gcc 9.3.0.

Reproducible example code

#include <pybind11/pybind11.h>

namespace py = pybind11;

void deprecation_warning(const char *msg)
{
    if (PyErr_WarnEx(PyExc_DeprecationWarning, msg, 1) == -1)
        throw py::error_already_set();
}

class A
{
};

PYBIND11_MODULE(pybind11_property, m) {
    py::class_<A>(m, "A")
        .def_property_readonly_static("FOO",
            [](py::object) {
                deprecation_warning("FOO is deprecated");
                return 3;
            }
        )
    ;
}

When run with python -c 'from pybind11_property import A; A.FOO' I get

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "<string>", line 4, in __get__
KeyError: '__builtins__'

In case it's useful, here are the pyproject.toml and setup.py I'm using to install the module (with pip install -e .):

[build-system]
requires = ["setuptools", "wheel", "pybind11==2.6.2"]

#!/usr/bin/env python
from setuptools import setup
from pybind11.setup_helpers import Pybind11Extension

setup(
    name='pybind11_property',
    version='0.1',
    ext_modules=[Pybind11Extension(
        name='pybind11_property',
        sources=['property.cpp'])]
)

Start with the reproducible property.cpp example and the pybind11 def_property_readonly_static entry point. Run the example under the stated PyPy 7.3.5 and pybind11 2.6.2 versions, then compare the PyErr_WarnEx path with the failing KeyError traceback. Done means identifying and resolving the PyPy-specific failure with a regression test.