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* fix: respect PYTHON_VERSION if set in classic mode

* fix: add warning when using PYTHON_VERSION

* fix: reduce target collision in add_submodule mode

Closes #2420

* fix: update CMakeLists.txt

CPython configured with `--with-pydebug` could not use `pybind11_add_module`

* 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

fix: include PYTHON_IS_DEBUG

- 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.

This variable is a CMake community standard to set the C++
standard of a build. Document it in favor of the previous variable,
which stays as a legacy flag for existing projects.

https://cmake.org/cmake/help/v3.17/variable/CMAKE_CXX_STANDARD.html

Add Python 3.8 to considered versions in CMake for additional
hints.
https://cmake.org/cmake/help/v3.2/module/FindPythonLibs.html