* Update pytest to 6.2.1 in tests/requirements.txt

* Pin pytest to last supported version for 3.5

* Suppress PytestUnraisableExceptionWarning and use sys.__unraisablehook__ instead of sys.unraisablehook

* Fix filterwarnings mark on old pytest and old Python versions

* Cleanup ignore_pytest_unraisable_warning decorator

* docs: mention PYTHONPATH in installing.rst

When pybind11 is included as a submodule, the user needs to update their
Python module search path.  Otherwise, the first c++ compilation command
in docs/basics.rst will fail.

* docs: add a note about compiling the example

This note shows how to modify the compilation command for the example
when the pybind11 source has been included as a Git submodule.

* docs: add a note about compiling the example

Added an internal link to the docs

* docs: updated a note about compiling the example

Also updated the command substitution syntax for consistency

... by leaving the unused parameters anonymous.

(This can be checked e.g. by compiling python_example with -Wall
-Wextra.)

* Avoid thread termination in scoped_released

Do not call `PyEval_RestoreThread()` from `~gil_scoped_release()` if python runtime is finalizing, as it will result in thread termination in Python runtime newer than 3.6, as documented in https://docs.python.org/3/c-api/init.html#c.PyEval_RestoreThread
Similarly do not call `PyThreadState_DeleteCurrent` from `~gil_scoped_acquire()` if runtime is finalizing.

Discovered while debugging PyTorch crash using Python-3.9 described in  https://github.com/pytorch/pytorch/issues/47776

* Simplify _Py_IsFinalizing() availability check

* Fix typo

* Add version agnostic `detail::finalization_guard()`

* Move `finalization_guard` to detail/common.h

And rename it to `is_finalizing`

* Move `is_finalizing()` back to pybind11.h

* Simplify `is_finalizing()` check

One should follow documentation rather than make any assumptions

* feat: disarm

* docs: fix comment

Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>

* minor cleanup: fixing or silencing flake8 errors

* ci: lock CMake to non-Universal version

* Update .github/workflows/ci.yml

Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>

fix: also throw in the move-constructor added by the PYBIND11_OBJECT macro, after the argument has been moved-out (if necessary) (#2701)

which was just before set to True in instance->allocate_layout()

* fix: regression with installed pybind11 overriding discovered one

Closes #2709

* docs: wording incorrect

* ci: drop pypy2 linux, add Python 10 dev

* ci: fix mistake

* ci: commented-out PGI 20.11, drop 20.7

Adjusting `type_caster<std::reference_wrapper<T>>` to support const/non-const propagation in `cast_op`. (#2705)


* Allow type_caster of std::reference_wrapper<T> to be the same as a native reference.

Before, both std::reference_wrapper<T> and std::reference_wrapper<const T> would
invoke cast_op<type>. This doesn't allow the type_caster<> specialization for T
to distinguish reference_wrapper types from value types.

After, the type_caster<> specialization invokes cast_op<type&>, which allows
reference_wrapper to behave in the same way as a native reference type.

* Add tests/examples for std::reference_wrapper<const T>

* Add tests which use mutable/immutable variants

This test is a chimera; it blends the pybind11 casters with a custom
pytype implementation that supports immutable and mutable calls.

In order to detect the immutable/mutable state, the cast_op needs
to propagate it, even through e.g. std::reference<const T>

Note: This is still a work in progress; some things are crashing,
which likely means that I have a refcounting bug or something else
missing.

* Add/finish tests that distinguish const& from &

Fixes the bugs in my custom python type implementation,
demonstrate test that requires const& and reference_wrapper<const T>
being treated differently from Non-const.

* Add passing a const to non-const method.

* Demonstrate non-const conversion of reference_wrapper in tests.

Apply formatting presubmit check.

* Fix build errors from presubmit checks.

* Try and fix a few more CI errors

* More CI fixes.

* More CI fixups.

* Try and get PyPy to work.

* Additional minor fixups. Getting close to CI green.

* More ci fixes?

* fix clang-tidy warnings from presubmit

* fix more clang-tidy warnings

* minor comment and consistency cleanups

* PyDECREF -> Py_DECREF

* copy/move constructors

* Resolve codereview comments

* more review comment fixes

* review comments: remove spurious &

* Make the test fail even when the static_assert is commented out.

This expands the test_freezable_type_caster a bit by:
1/ adding accessors .is_immutable and .addr to compare identity
from python.
2/ Changing the default cast_op of the type_caster<> specialization
to return a non-const value. In normal codepaths this is a reasonable
default.
3/ adding roundtrip variants to exercise the by reference, by pointer
and by reference_wrapper in all call paths.  In conjunction with 2/, this
demonstrates the failure case of the existing std::reference_wrpper conversion,
which now loses const in a similar way that happens when using the default cast_op_type<>.

* apply presubmit formatting

* Revert inclusion of test_freezable_type_caster

There's some concern that this test is a bit unwieldly because of the use
of the raw <Python.h> functions. Removing for now.

* Add a test that validates const references propagation.

This test verifies that cast_op may be used to correctly detect
const reference types when used with std::reference_wrapper.

* mend

* Review comments based changes.

1. std::add_lvalue_reference<type> -> type&
2. Simplify the test a little more; we're never returning the ConstRefCaster
type so the class_ definition can be removed.

* formatted files again.

* Move const_ref_caster test to builtin_casters

* Review comments: use cast_op and adjust some comments.

* Simplify ConstRefCasted test

I like this version better as it moves the assertion that matters
back into python.

This is nice enough to be mentioned explicitly in the docs.

docs: add warning about FindPython's Development component when libraries don't exist (e.g. on manylinux) (#2689)

* Add waring about FindPython's Development component when libraries don't exist (e.g. on manylinux)

* Minor wording update (thanks, @henryiii!)

`git submodule add` needs the branch before the repository or else it is ignored. The previous code checked out the `master` branch, not the `stable` branch.

* List all the pyind11 exceptions.

For curious readers, see `translate_exception` and
`PYBIND11_RUNTIME_EXCEPTION.

* Added test_thread testing for ostream_redirect segfault recreation

* fix: scoped_ostream_redirect str created outside gil

* Moved threading tests into test_iostream. Cleaned up some formatting. Deleted test_thread.{cpp,py}

* CI: few formatting fixes

* CI: yet another formatting fix

* CI: more formatting fixes. Removed unecessary comment

* Ignore 'warning C4702: unreachable code' in MSVC 2015

Co-authored-by: Nick Bridge <nick.bridge.chess@gmail.com>
Co-authored-by: Nick Bridge <nbridge@jumptrading.com>
Co-authored-by: Yannick Jadoul <yannick.jadoul@belgacom.net>

* feat: lazy compile

* refactor: lazy -> only_changed

* refactor: leave the changed function up to the user

* refactor: pass a function, based on @YannickJadoul and @HDembinski's suggestions

* refactor: old -> _old, as it's not intended for users

* docs: slight improvmenent from @rwgk

* docs: Ccache spelling, extra warning about pip caching

Ccache spelling noted by @YannickJadoul

* Add argument names to enum_ methods

* Add test_enum::test_docstring_signatures

* style: clang-tidy: modernize-use-using

* style: more clang-tidy checking

Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>

* fix: match new extension discovery with changes to classic discovery

Followup to #2638 - this was fixed in 2.6.0, but only for classic Python

* fix: followup to avoid warnings