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* Use sysconfig in Python >= 3.10

Rely on sysconfig for installation paths for Python >= 3.10. distutils
has been deprecated and will be removed.

Fixes: #3677

* Explicitly select the posix_prefix scheme for platinclude on Debian

Debian's default scheme is posix_local, for installing locally-built
packages to /usr/local/.  We want to find the Python headers in /usr/,
so search posix_prefix.

* style: avoid using unintialized variables

Tested with cmake --warn-unintialized -S . -B build

* refactor: use function for possibly uninit vars

* 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

* feat: FindPython support

* refactor: rename to PYBIND11_FINDPYTHON

* docs: Caps fixes

* feat: NOPYTHON mode

* test: check simple call

* docs: add changelog/upgrade guide

* feat: Support Python3 and Python2

* refactor: Use targets in tests

* fix: support CMake 3.4+

* feat: classic search also finds virtual environments

* docs: some updates from @wjakob's review

* fix: wrong name for QUIET mode variable, reported by @skoslowski

* refactor: cleaner output messaging

* fix: support debug Python's in FindPython mode too

* fixup! refactor: cleaner output messaging

* fix: missing pybind11_FOUND and pybind11_INCLUDE_DIR restored to subdir mode

* fix: nicer reporting of Python / PyPy

* fix: out-of-order variable fix

* docs: minor last-minute cleanup

- look for windows python lib when using mingw & msys
  - if not found, then look for system python lib as before

pybind11_add_module fails because FindPythonLibsNew.cmake try to link PythonXX.lib, but the first letter "p" should be lowercase.

Since the module is named FindPythonLibsNew, PythonLibsNew_FOUND should be set appropriately.

Found via `codespell`

Make sure `LibsNew` runs correctly if called after the old `Libs`.

* Use LIBDIR and MULTIARCH on linux to find python library

* Remove apple-specific setting; the non-windows one should work fine on OS X

* Default LIBDIR/MULTIARCH to '' (to avoid getting None)

* Remove trailing whitespace from FindPythonLibsNew

Fixes #365. `sysconfig.get_config_var('SO')` already returns the correct
PYTHON_MODULE_EXTENSION, even for debug builds, so there is no need to
add anything else manually.

When run on windows in a venv, PYTHON_LIBRARY pointet to a non-existant
location in the virtual environment directory.

This has been fixed by testing if the path exists and, if not, trying
an alternative path, relative to the PYTHON_INCLUDE_DIR.

If the alternative path doesn't exit as well, an error will be raised.

Also:
- Update documented result variables
- Remove long-ago-deprecated variable