fix(cmake): unset stale PYTHON_MODULE_DEBUG_POSTFIX and correct USE_PYTHON_INCLUDE_DIR variable (#6086)

When the Python executable changes between CMake runs,
PYTHON_MODULE_DEBUG_POSTFIX was not being unset alongside PYTHON_IS_DEBUG
and PYTHON_MODULE_EXTENSION, allowing a stale postfix to survive interpreter
changes.

In the USE_PYTHON_INCLUDE_DIR block, the elseif branch checked
PYTHON_INCLUDE_DIR (singular) but expanded PYTHON_INCLUDE_DIRS (plural),
always silently resolving to an empty string. Correct it to expand
${PYTHON_INCLUDE_DIR}. Also add a Python3_INCLUDE_DIRS branch so the
option works when pybind11NewTools resolves Python via the Python3
find-package component.

Assisted-by: ClaudeCode:claude-fable-5

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index bca0c22..3abb3dc 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -304,8 +304,10 @@
 # Relative directory setting
 if(USE_PYTHON_INCLUDE_DIR AND DEFINED Python_INCLUDE_DIRS)
   file(RELATIVE_PATH CMAKE_INSTALL_INCLUDEDIR ${CMAKE_INSTALL_PREFIX} ${Python_INCLUDE_DIRS})
+elseif(USE_PYTHON_INCLUDE_DIR AND DEFINED Python3_INCLUDE_DIRS)
+  file(RELATIVE_PATH CMAKE_INSTALL_INCLUDEDIR ${CMAKE_INSTALL_PREFIX} ${Python3_INCLUDE_DIRS})
 elseif(USE_PYTHON_INCLUDE_DIR AND DEFINED PYTHON_INCLUDE_DIR)
-  file(RELATIVE_PATH CMAKE_INSTALL_INCLUDEDIR ${CMAKE_INSTALL_PREFIX} ${PYTHON_INCLUDE_DIRS})
+  file(RELATIVE_PATH CMAKE_INSTALL_INCLUDEDIR ${CMAKE_INSTALL_PREFIX} ${PYTHON_INCLUDE_DIR})
 endif()
 
 if(PYBIND11_INSTALL)

tools/pybind11NewTools.cmake

diff --git a/tools/pybind11NewTools.cmake b/tools/pybind11NewTools.cmake
index 5ab3142..568b0b0 100644
--- a/tools/pybind11NewTools.cmake
+++ b/tools/pybind11NewTools.cmake
@@ -112,6 +112,7 @@
     # Detect changes to the Python version/binary in subsequent CMake runs, and refresh config if needed
     unset(PYTHON_IS_DEBUG CACHE)
     unset(PYTHON_MODULE_EXTENSION CACHE)
+    unset(PYTHON_MODULE_DEBUG_POSTFIX CACHE)
   endif()
 
   set(PYBIND11_PYTHON_EXECUTABLE_LAST