* Add debug fprintf to test_interpreter.cpp

* Update `sys.path` from `PYTHONPATH` in Python >= 3.11 branch of `initialize_interpreter()`

* Use `config.isolated = 0; config.use_environment = 1;`

As suggsted by @vstinner here: https://github.com/pybind/pybind11/pull/4119#issuecomment-1219442853

* Add `TEST_CASE("PYTHONPATH is used to update sys.path")`

* Fix clang-tidy error.

* Use `_putenv_s()` under Windows.

* Fix clang-tidy error: argument name ... in comment does not match parameter name

* Remove slash from PYTHONPATH addition, to work around Windows slash-vs-backslash issue.

* Use `py::str(...)` instead of `.attr("__str__")` as suggested by @skylion007

Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>

Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>