* Demonstrate test_factory_constructors.py failure without functional changes from #2335

* Revert "Demonstrate test_factory_constructors.py failure without functional changes from #2335"

This reverts commit ca33a8021fc2a3617c3356b188796528f4594419.

* Fix test crash where registered Python type gets garbage collected

* Clean up some more internal structures when class objects go out of scope

* Reduce length of std::erase_if-in-C++20 comment

* Clean up code for cleaning up type internals

* Move cleaning up of type info in internals to tp_dealloc on pybind11_metaclass

* style: use Black everywhere

* style: minor touchup from review

* Check if valgrind-detected after-freeing access of PyMethodDef causes macOS Python 3.9 segfaults

* fix: only apply leak on 3.9.0

* fix: faster check

* fix: better naming thanks to @bstaletic

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

* feat: basic typing support

* docs: mention syncing as suggested by @rwgk

* docs: update changelog

* docs: copy of warning in limitations

* fix: ipo should be off for debug or relwithdebinfo

Closes #2587

* docs: slightly more detailed about IPO

* Update pybind11Common.cmake

Fix minor bug in `_add_lflags` that allows a flag to be added more than once.

* docs: some final 2.6.0 updates

* docs: better warning text

Closes #2571; this is defined to share, and unlike DATADIR, is not sometimes overwritten incorrectly for storing CMake files.

* Throw error_already_set in py::len on failing PyObject_Length

* Fix tests to mach error message on PyPy

Shorten PYBIND11_EMBEDDED_MODULE macro implementation by using PYBIND11_CATCH_INIT_EXCEPTIONS (#2579)

* fix: Intel 18+ fully supported

* fix: Intel compiler workaround no longer needed

Followup on #94 now that Intel 18+ is required.

* add support for LLVM compiler

* Update include/pybind11/pybind11.h

__forced_unwind libc++ vs libstdc++

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

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

* Update PGI to 20.9

Update PGI to 20.9

* Update ci.yml

* Update ci.yml

* Update README.rst

* Update ci.yml

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

* docs: FAQ CMake updates

* docs: limitations

* ci: don't over label

* docs: update CHANGELOG, add a bit more structure

* ci: label PRs with more labels, and sooner

* docs: updates from @rwgk

* docs: address @YannickJadoul's points