feat!: drop support for Python 3.8, MSVC 2017 (#6110)

* feat!: drop support for Python 3.8

The minimum supported version is now Python 3.9. pybind11 v3.0 was the last
release that supports Python 3.8. The deprecation note said that support goes
away in 3.1.

Remove the code paths that this makes dead:

- the `_PyObject_Vectorcall` fallback in `cast.h`
- the `frame->f_code` and `frame->f_back` fallbacks in `pytypes.h`
- the `PyFrame_FastToLocals` path in `get_type_override`
- the conditional `Py_VISIT(Py_TYPE(self))` in `tp_traverse`
- `PYBIND11_PYCONFIG_SUPPORT_PY_VERSION_HEX`, the pre-PyConfig interpreter
  init, and the `widen_chars` helpers in `embed.h`

Assisted-by: ClaudeCode:claude-opus-5

* ci(appveyor): use Python 3.9

The AppVeyor job set `PYTHON: 38`, which makes the path `C:\Python38`.
The image gives Python 3.9.13 as `C:\Python39`.

Assisted-by: ClaudeCode:claude-opus-5

* feat!: require MSVC 2019 or newer

Python 3.9 is the new minimum, so MSVC 2017 is no longer needed. Raise the
compile-time floor to _MSC_VER 1920 and remove the workarounds that only
applied below it: std::launder, fold expressions, weak_from_this, aligned
new/delete, the C4100 warning helper, and the func_handle syntax error.

AppVeyor now builds with Visual Studio 2019.

Assisted-by: ClaudeCode:claude-opus-5

* fix(appveyor): build against the Python that has the test packages

CMake 4 has no FindPythonLibs, so pybind11 uses FindPython. FindPython reads
the registry before PATH and selected `C:\Python314-x64`, but the test packages
go into the Python on PATH. Pass `Python_ROOT_DIR` to name the correct one.

Also set `CMAKE_ARCH`. It was never set, so the architecture came from the
generator. Visual Studio 2017 defaults to Win32, but Visual Studio 2019
defaults to x64, which made this x86 job build 64-bit code.

Assisted-by: ClaudeCode:claude-opus-5

* fix(appveyor): give the linker Python's libs directory

The build compiled but failed to link with LNK1104 on a bare `python39.lib`.
That name comes from the `#pragma comment(lib, ...)` in pyconfig.h, so the
linker needs the directory. CMake 4 has no FindPythonLibs, and FindPython does
not add it for this Debug x86 build. Put it on LIB instead.

The directory listing is temporary, to confirm the library is present.

Assisted-by: ClaudeCode:claude-opus-5

* fix(appveyor): link the release Python library in Debug

The image ships python39_d.lib next to python39.lib, so FindPython picks the
debug import library for a Debug build. pybind11 undefines _DEBUG around
Python.h, so pyconfig.h asks for python39.lib instead and the link failed with
LNK1104. Name the release library for the debug slot.

Setting LIB does not work, because MSBuild replaces it from the toolset, and it
would link both import libraries.

Assisted-by: ClaudeCode:claude-opus-5

* chore(appveyor): print link settings to debug LNK1104

Revert the two attempted fixes. Neither changed the failure: setting LIB does
not survive MSBuild, and naming the release library for Python_LIBRARY_DEBUG
had no effect.

Print the Python cache entries and the link settings of a generated project
file instead, to see what the linker really gets. Temporary.

Assisted-by: ClaudeCode:claude-opus-5

* fix(appveyor): take the release Python library in Debug

The generated project file linked C:\Python39\libs\python39_d.lib in the Debug
configuration, because the image ships debug binaries next to the release ones.
pybind11 undefines _DEBUG around Python.h, so pyconfig.h asks for python39.lib
in a #pragma comment(lib), which nothing on the link line satisfies and no
library directory holds. Map Debug to the release artifacts.

Assisted-by: ClaudeCode:claude-opus-5

* chore(appveyor): drop the temporary link diagnostic

Assisted-by: ClaudeCode:claude-opus-5

* fix(tests): guard the unraisable warning filter for pytest < 6

The distro pytest in the Clang and GCC Docker jobs has no
PytestUnraisableExceptionWarning, so an unconditional filterwarnings
mark makes pytest fail with an INTERNALERROR after the tests pass.

Assisted-by: ClaudeCode:claude-opus-5

Claude-Session: https://claude.ai/code/session_01Aimf6HuSz1vLRwBnbxmCTc

* docs: address review items on version hints, embed docs, and a PyPy xfail

Extend Python_ADDITIONAL_VERSIONS through 3.15, describe the PyConfig
behavior of initialize_interpreter, and drop the stale Python 3.8 wording
from the PyPy xfail reason.

Assisted-by: ClaudeCode:claude-opus-5

* Update README.rst

Co-authored-by: Ralf W. Grosse-Kunstleve <rwgkio@gmail.com>

---------

Co-authored-by: Ralf W. Grosse-Kunstleve <rwgkio@gmail.com>