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

* Adding missing virtual destructors, to silence clang -Wnon-virtual-dtor warnings.

Tested with clang version 9.0.1-12 under an Ubuntu-like OS.
Originally discovered in the Google-internal environment.

* adding -Wnon-virtual-dtor for GNU|Intel|Clang

* Add __builtins__ to globals argument of `py::exec` and `py::eval` if not present

* Refactor into inline ensure_builtins_in_globals function

* ci: add more jobs

* ci: minor trimdown