fix(clang-tidy): Add cppcoreguidelines-init-vars,slicing, and throw-by-value-catch-by-reference checks (#3094)

* clang-tidy: guard against more UB behavior

* Remove slicing check for now

* docs: update changelog

* docs: add one more and merge tidy

* Adding test_return_vector_bool_raw_ptr to test_stl.py.

* First attempt to make the documentation more accurate, but not trying to be comprehensive, to not bloat the reference table with too many details.

* Fixing minor oversights.

* Applying reviewer suggestion.

* 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]

* 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.

* Convert PyDict_GetXXX to internal error checking variant

* Check unlikely error return from PyDict_DelItemString

* pickle setstate: setattr __dict__ only if not empty, to not force use of py::dynamic_attr() unnecessarily.

* Adding unit test.

* Clang 3.6 & 3.7 compatibility.

* PyPy compatibility.

* Minor iwyu fix, additional comment.

* Addressing reviewer requests.

* Applying clang-tidy suggested fixes.

* Adding check_dynamic_cast_SimpleCppDerived, related to issue #3062.

* Allow casting from None to a custom object, closes #2778

* ci.yml patch from the smart_holder branch for full CI coverage.

* Updating and slightly enhancing instructions for running clang-tidy.

* Making documented commands identical to workflow commands. Adding comment in workflow file pointing to documentation.

* Initial fixes

* Whoops

* Finish clang-tidy manual fixes

* Add two missing fixes

* Revert

* Update clang-tidy

* Try to fix unreachable code error

* Move nolint comment

* Apply missing fix

* Don't override clang-tidy config

* Does this fix clang-tidy?

* Make all clang-tidy errors visible

* Add comments about NOLINTs and remove a few

* Fix typo

* Enable and apply clang-tidy readability and misc fixes.

* Revert deprecated tester

* add space to tests/test_constants_and_functions.cpp

* Add more modernize clang-tidy checks

* Enable a few more

* Apply performance fixes from clang-tidy

* 2nd Round of Perf Optimizations

* 3rd round of fixes & handle false-positive

* Apply missing fix and clang-format

* Apply reviewer comment

* Experiment using -DCMAKE_BUILD_TYPE=Debug for Centos 8.

* Moving comment because the current placement seems to mess up the cmake command.

* Using `echo > VAR_XXX` and `$(cat VAR_XXX)` trick to emulate using variables (https://github.com/actions/starter-workflows/issues/68#issuecomment-524937002).

* Add const T to docstring generation.

* Change order.

* See if existing test triggers for a const type.

* Add tests.

* Fix test.

* Remove experiment.

* Reformat.

* More tests, checks run.

* Adding `test_fmt_desc_` prefix to new test functions.

* Using pytest.mark.parametrize to 1. condense test; 2. exercise all functions even if one fails; 3. be less platform-specific (e.g. C++ float is not necessarily float32).

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