[BUG] test_builtin_caster.test_int_convert: Fails on certain minor version of CPython 3.8?

[EricCousineau-TRI]

Issue description

Test fails on current mainline branch:

pybind11/tests/test_builtin_casters.py

Lines 302 to 305 in 0e01c24

	# TODO: Avoid DeprecationWarning in `PyLong_AsLong` (and similar)
	if (3, 8) <= env.PY < (3, 10):
	with pytest.deprecated_call():
	assert convert(Int()) == 42

Reproducible example code

$ cd pybind11
$ git log -n 1 --oneline --no-decorate
0e01c243 Generalizing suppression for pypocketfft. (#2896)
$ mkdir build && cd build
$ cmake .. -GNinja -DPYBIND11_PYTHON_VERSION=3.8 -DPYBIND11_TEST_OVERRIDE=test_builtin_casters.cpp
...
-- PYTHON 3.8.0
...
$ env PYTEST_ADDOPTS="-s -x" ninja pytest
...
        # TODO: Avoid DeprecationWarning in `PyLong_AsLong` (and similar)
        if (3, 8) <= env.PY < (3, 10):
            with pytest.deprecated_call():
>               assert convert(Int()) == 42
E               AssertionError: Did not produce DeprecationWarning or PendingDeprecationWarning

[EricCousineau-TRI]

Changed title. This passes w/ CPython 3.6.9.

However, it still fails on Ubuntu 18.04's flavor (3.8.0). This happens b/c pybind11 picked up latest version on my system.

[EricCousineau-TRI]

Per #2923, it's b/c Debian ships old version (ofc).

$ cat /etc/issue
Ubuntu 18.04.5 LTS \n \l
$ python3.8 --version
Python 3.8.0
$ python3.8 -m pytest --version
This is pytest version 3.3.2, imported from /usr/lib/python3/dist-packages/pytest.py
...

[EricCousineau-TRI]

PR basically uses a narrow version of this pytest-dev/pytest#4104