* chore: add nox support

* chore: add more lines to CODEOWNERS

Fixing spelling errors that went undetected because the pre-commit spell check was added after the CI for PR #2995 last ran. (#3103)

* Adding iostream.h thread-safety documentation.

* Restoring `TestThread` code with added `std::lock_guard<std::mutex>`.

* Updating new comments to reflect new information.

* Fixing up `git rebase -X theirs` accidents.

* Copying from prework_no_rst branch (PR #3087): test_numpy_array.cpp, test_stl.cpp

* Manual changes reducing NOLINTs.

* clang-format-diff.py

* Minor adjustment to avoid MSVC warning C4702: unreachable code

[skip ci]

* * Rollback of PR #3068.
* Using latest pytest main branch for 3.9 and 3.10.
* WORKAROUND_ENABLING_ROLLBACK_OF_PR3068 in test_class_sh_trampoline_shared_from_this.py

First experiment combining two potential fixes: latest pytest, workaround.
If this succeeds the next step will be to try only latest pytest without the workaround.

Note: the workaround is known to resolve the MSAN error reported under
https://github.com/pybind/pybind11/pull/3068#issuecomment-877658470

* WORKAROUND_ENABLING_ROLLBACK_OF_PR3068 = False

* Narrowing down WORKAROUND_ENABLING_ROLLBACK_OF_PR3068 to Python 3.9

* py::pickle typo

* correct dots and parentheses

Go all the way fixing clang-tidy issues to avoid the NOLINTNEXTLINE clutter and clang-format issues. This was really meant to be part of PR #3051 but was held back either out of an abundance of caution, or because of confusion caused by stray semicolons. (#3086)

* Enable clang-tidy else-after-return and redundant void checks

* Fix remaining else-after

* Address reviewer comments

* Fix indentation

* Rerun clang-tidy post merge

* Splitting out pybind11/stl/filesystem.h.

To solve breakages like: https://github.com/deepmind/open_spiel/runs/2999582108

Mostly following the suggestion here: https://github.com/pybind/pybind11/pull/2730#issuecomment-750507575

Except using pybind11/stl/filesystem.h instead of pybind11/stlfs.h, as decided via chat.

stl.h restored to the exact state before merging PR #2730 via:
```
git checkout 733f8de2 stl.h
```

* Properly including new stl subdirectory in pip wheel config.

This now passes interactively:
```
pytest tests/extra_python_package/
```

* iwyu cleanup.

iwyuh.py -c -std=c++17 -DPYBIND11_TEST_BOOST -Ipybind11/include -I/usr/include/python3.9 -I/usr/include/eigen3 include/pybind11/stl/filesystem.h

* Adding PYBIND11_HAS_FILESYSTEM_IS_OPTIONAL.

* Eliminating else after return.

* Unify cast_error message thrown by [simple|unpacking]_collector

simple_collector and unpacking_collector throw different error messages
when the casting of an argument failed: While the former mentions make_tuple(),
the latter emphasises the call argument (and its name/position).

* Consolidating "Unable to convert call argument" error reporting code to guarantee uniformity.

Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>

Found via `codespell -q 3 -L nd,ot,thist`

Also removing a commented-out line.

Also removing a commented-out line.

* Convert PyDict_GetXXX to internal error checking variant

* Check unlikely error return from PyDict_DelItemString

I did not go back to check when and why exactly this slipped in.

Also removing unused `msg` (oversight in PR #3059).