* 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 733f8de24f 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.
* feat: setup.py redesign and helpers
* refactor: simpler design with two outputs
* refactor: helper file update and Windows support
* fix: review points from @YannickJadoul
* refactor: fixes to naming and more docs
* feat: more customization points
* feat: add entry point pybind11-config
* refactor: Try Extension-focused method
* refactor: rename alt/inplace to global
* fix: allow usage with git modules, better docs
* feat: global as an extra (@YannickJadoul's suggestion)
* feat: single version location
* fix: remove the requirement that setuptools must be imported first
* fix: some review points from @wjacob
* fix: use .in, add procedure to docs
* refactor: avoid monkeypatch copy
* docs: minor typos corrected
* fix: minor points from @YannickJadoul
* fix: typo on Windows C++ mode
* fix: MSVC 15 update 3+ have c++14 flag
See <https://docs.microsoft.com/en-us/cpp/build/reference/std-specify-language-standard-version?view=vs-2019>
* docs: discuss making SDists by hand
* ci: use pep517.build instead of manual setup.py
* refactor: more comments from @YannickJadoul
* docs: updates from @ktbarrett
* fix: change to newly recommended tool instead of pep517.build
This was intended as a proof of concept; build seems to be the correct replacement.
See https://github.com/pypa/pep517/pull/83
* docs: updates from @wjakob
* refactor: dual version locations
* docs: typo spotted by @wjakob
