Resolve PyPy 7.3.22 `PyType_Ready` import failure in PyPy 3.11 CI on macOS and Windows

[rwgk]

The macOS and Windows PyPy 3.11 CI jobs started failing on current master, while the same jobs were still green in PR #6036, which is the last known full CI run we checked.

The most important observation from the logs is that:

• the last known working macOS/Windows PyPy 3.11 jobs used PyPy 7.3.21 (Py 3.11.15)
• the current failing macOS/Windows PyPy 3.11 jobs use PyPy 7.3.22 (Py 3.11.15)
• Ubuntu PyPy 3.11 stayed on PyPy 7.3.21 (Py 3.11.15) and still passed

That strongly suggests either:

1. a regression in PyPy 7.3.22, or
2. a pybind11 compatibility issue that is exposed by PyPy 7.3.22

As an immediate mitigation, we should pin the macOS and Windows PyPy 3.11 CI jobs to pypy-3.11-v7.3.21 (PR #6050).

Last known working CI

PR #6036

• macOS PyPy 3.11 success:
  • https://github.com/pybind/pybind11/actions/runs/24612330326/job/71968766480?pr=6036
• Windows PyPy 3.11 success:
  • https://github.com/pybind/pybind11/actions/runs/24612330326/job/71968766475?pr=6036

Current failing CI

PR #5850 (master_with_empty_commit, rebased on current master)

• macOS PyPy 3.11 failure:
  • https://github.com/pybind/pybind11/actions/runs/25258404339/job/74063820293?pr=5850
• Windows PyPy 3.11 failure:
  • https://github.com/pybind/pybind11/actions/runs/25258404339/job/74063820280?pr=5850

Ubuntu observation

Ubuntu PyPy 3.11 did not reproduce the failure in the runs we checked, but the logs show that it was still using PyPy 7.3.21, so this is not evidence that 7.3.22 is fine on Ubuntu.

• PR build: support Eigen 5 #6036 Ubuntu PyPy 3.11 success:
  • https://github.com/pybind/pybind11/actions/runs/24612330326/job/71968766459?pr=6036
• PR Empty commit on top of master. Just to trigger CI. #5850 Ubuntu PyPy 3.11 success:
  • https://github.com/pybind/pybind11/actions/runs/25258404339/job/74063820431?pr=5850

Relevant log observations:

• PR build: support Eigen 5 #6036 macOS success used PyPy 7.3.21 (Py 3.11.15)
• PR build: support Eigen 5 #6036 Windows success used PyPy 7.3.21 (Py 3.11.15)
• PR Empty commit on top of master. Just to trigger CI. #5850 Ubuntu success used PyPy 7.3.21 (Py 3.11.15)
• PR Empty commit on top of master. Just to trigger CI. #5850 macOS failure used PyPy 7.3.22 (Py 3.11.15)
• PR Empty commit on top of master. Just to trigger CI. #5850 Windows failure used PyPy 7.3.22 (Py 3.11.15)

So the current data is consistent with:

• 7.3.21: known good on macOS, Windows, Ubuntu
• 7.3.22: known bad on macOS, Windows
• 7.3.22 on Ubuntu: not yet tested / not established from these runs

Error details from the failing logs

The macOS and Windows failures are effectively identical. In both cases, pytest fails immediately while importing pybind11_tests from tests/conftest.py.

macOS:

Traceback (most recent call last):
  File "/Users/runner/work/pybind11/pybind11/tests/conftest.py", line 26, in <module>
    import pybind11_tests
ImportError: D: PyType_Ready failed: TypeError: instance layout conflicts in multiple inheritance
ImportError while loading conftest '/Users/runner/work/pybind11/pybind11/tests/conftest.py'.
E   ImportError: D: PyType_Ready failed: TypeError: instance layout conflicts in multiple inheritance
FAILED: [code=4] tests/CMakeFiles/pytest

Windows:

Traceback (most recent call last):
    File "D:\a\pybind11\pybind11\tests\conftest.py", line 26, in <module>
      import pybind11_tests
  ImportError: D: PyType_Ready failed: TypeError: instance layout conflicts in multiple inheritance
  ImportError while loading conftest 'D:\a\pybind11\pybind11\tests\conftest.py'.
  E   ImportError: D: PyType_Ready failed: TypeError: instance layout conflicts in multiple inheritance

Proposed action

• Pin only the macOS and Windows PyPy 3.11 jobs to 7.3.21 for now.
• Leave Ubuntu unpinned unless it also starts failing.
• Open a follow-up investigation into what changed between PyPy 7.3.21 and 7.3.22, or which pybind11 type registration/import path is now triggering:

PyType_Ready failed: TypeError: instance layout conflicts in multiple inheritance