Description of changes

Preparing for the version 3 release, I found a few fixes that are relevant even for version 2. Currently the build fails with:

Executing pytestCheckPhase
pytest flags: -m pytest
Traceback (most recent call last):
  File "/build/source/tests/conftest.py", line 25, in <module>
    import pybind11_tests
ModuleNotFoundError: No module named 'pybind11_tests'
ImportError while loading conftest '/build/source/tests/conftest.py'.
tests/conftest.py:25: in <module>
    import pybind11_tests
E   ModuleNotFoundError: No module named 'pybind11_tests'

See also:

• [3.0.0]: How to build `pybind11_tests` Python module for running `pytest`? (before distributing) pybind/pybind11#5741
• Migrate to Pybind11 v3 matplotlib/matplotlib#30291

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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