Fix broken pytest integration (including CI failure in src/conftest_test.py)

[mkoeppe]

Frequently seen in the CI, for example https://github.com/sagemath/sage/actions/runs/9055116851/job/24875749181?pr=37862#step:12:27

Doctesting files changed since last git commit
No files to doctest
Features detected for doctesting: 
Running pytest on ['/sage/src'] with options []
============================= test session starts ==============================
platform linux -- Python 3.10.12, pytest-8.2.0, pluggy-1.5.0
rootdir: /sage/src
configfile: tox.ini
plugins: xdist-3.6.1, mock-3.14.0, anyio-4.0.0, hypothesis-6.100.7
collected 63 items / 2 skipped

src/conftest_test.py .F                                                  [  3%]
src/sage/doctest/parsing_test.py ...........                             [ 20%]
src/sage/manifolds/differentiable/examples/symplectic_space_test.py .... [ [26](https://github.com/sagemath/sage/actions/runs/9055116851/job/24875749181?pr=37862#step:12:27)%]
                                                                         [ 26%]
src/sage/manifolds/differentiable/symplectic_form_test.py .............. [ 49%]
.....................                                                    [ 82%]
src/sage/manifolds/differentiable/tensorfield_paral_test.py .            [ 84%]
src/sage/numerical/backends/cvxopt_backend_test.py ..                    [ 87%]
src/sage/numerical/backends/glpk_backend_test.py ..                      [ 90%]
src/sage/numerical/backends/glpk_exact_backend_test.py ..                [ 93%]
src/sage/numerical/backends/interactivelp_backend_test.py ..             [ 96%]
src/sage/numerical/backends/ppl_backend_test.py ..                       [100%]

=================================== FAILURES ===================================
______________ TestPytestSageScript.test_invoke_on_inputtest_file ______________

self = <conftest_test.TestPytestSageScript object at 0x7f0d91ecdae0>

    def test_invoke_on_inputtest_file(self):
        result = subprocess.run(
            ["sage", "--pytest", input_file],
            capture_output=True,
            text=True,
        )
>       assert result.returncode == 1  # There are failures in the input test
E       assert 2 == 1
E        +  where 2 = CompletedProcess(args=['sage', '--pytest', PosixPath('/sage/src/conftest_inputtest.py')], returncode=2, stdout="======...n !!!!!!!!!!!!!!!!!!!!\n=============================== 1 error in 0.18s ===============================\n", stderr='').returncode

src/conftest_test.py:40: AssertionError
=========================== short test summary info ============================
FAILED src/conftest_test.py::TestPytestSageScript::test_invoke_on_inputtest_file
============== 1 failed, 62 passed, 2 skipped in 97.56s (0:01:[37](https://github.com/sagemath/sage/actions/runs/9055116851/job/24875749181?pr=37862#step:12:38)) ==============Running pytest on ['/sage/src'] with options []
============================= test session starts ==============================
platform linux -- Python 3.10.12, pytest-8.2.0, pluggy-1.5.0
rootdir: /sage/src
configfile: tox.ini
plugins: xdist-3.6.1, mock-3.14.0, anyio-4.0.0, hypothesis-6.100.7
collected 63 items / 2 skipped

src/conftest_test.py .F                                                  [  3%]
src/sage/doctest/parsing_test.py ...........                             [ 20%]
src/sage/manifolds/differentiable/examples/symplectic_space_test.py .... [ 26%]
                                                                         [ 26%]
src/sage/manifolds/differentiable/symplectic_form_test.py .............. [ 49%]
.....................                                                    [ 82%]
src/sage/manifolds/differentiable/tensorfield_paral_test.py .            [ 84%]
src/sage/numerical/backends/cvxopt_backend_test.py ..                    [ 87%]
src/sage/numerical/backends/glpk_backend_test.py ..                      [ 90%]
src/sage/numerical/backends/glpk_exact_backend_test.py ..                [ 93%]
src/sage/numerical/backends/interactivelp_backend_test.py ..             [ 96%]
src/sage/numerical/backends/ppl_backend_test.py ..                       [100%]

=================================== FAILURES ===================================
______________ TestPytestSageScript.test_invoke_on_inputtest_file ______________

self = <conftest_test.TestPytestSageScript object at 0x7f0d91ecdae0>

    def test_invoke_on_inputtest_file(self):
        result = subprocess.run(
            ["sage", "--pytest", input_file],
            capture_output=True,
            text=True,
        )
>       assert result.returncode == 1  # There are failures in the input test
E       assert 2 == 1
E        +  where 2 = CompletedProcess(args=['sage', '--pytest', PosixPath('/sage/src/conftest_inputtest.py')], returncode=2, stdout="======...n !!!!!!!!!!!!!!!!!!!!\n=============================== 1 error in 0.18s ===============================\n", stderr='').returncode

src/conftest_test.py:40: AssertionError
=========================== short test summary info ============================
FAILED src/conftest_test.py::TestPytestSageScript::test_invoke_on_inputtest_file
============== 1 failed, 62 passed, 2 skipped in 97.56s (0:01:[37](https://github.com/sagemath/sage/actions/runs/9055116851/job/24875749181?pr=37862#step:12:38)) ==============

• Fixes pytest configuration broken with pytest 8.x; failures in Build & Test CI #37536.

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit 8c9a5a0; changes) is ready! 🎉
This preview will update shortly after each push to this PR.

[NathanDunfield]

Looks good to me.

[mkoeppe]

Thank you!