CMake: react to python version changes

The new FindPython-based variant of the CMake scripts caches information
about the chosen Python version that can become stale. For example,
suppose I configure a simple pybind11-based project as follows

```
cmake -S . -B build -GNinja -DPython_ROOT=<path to python 3.8>
```

which will generate `my_extension.cpython-38-x86_64-linux-gnu.so`.
A subsequent change to the python version like

```
cmake -S . -B build -GNinja -DPython_ROOT=<path to python 3.9>
```

does not update all necessary build system information. In particular,
the compiled file is still called
`my_extension.cpython-38-x86_64-linux-gnu.so`.

This commit fixes the problem by detecting changes in
`Python_EXECUTABLE` and re-running Python as needed.

Note that the previous way of detecting Python does not seem to be
affected, it always specifies the right suffix.

tools/pybind11NewTools.cmake

@@ -82,6 +82,15 @@ if(NOT DEFINED ${_Python}_EXECUTABLE)
 
 
 endif()
 endif()
 
 
+if(NOT ${_Python}_EXECUTABLE STREQUAL PYTHON_EXECUTABLE_LAST)
+  # Detect changes to the Python version/binary in subsequent CMake runs, and refresh config if needed
+  unset(PYTHON_IS_DEBUG CACHE)
+  unset(PYTHON_MODULE_EXTENSION CACHE)
+  set(PYTHON_EXECUTABLE_LAST
+      "${${_Python}_EXECUTABLE}"
+      CACHE INTERNAL "Python executable during the last CMake run")
+endif()
+
 if(NOT DEFINED PYTHON_IS_DEBUG)
 if(NOT DEFINED PYTHON_IS_DEBUG)
   # Debug check - see https://stackoverflow.com/questions/646518/python-how-to-detect-debug-Interpreter
   # Debug check - see https://stackoverflow.com/questions/646518/python-how-to-detect-debug-Interpreter
   execute_process(
   execute_process(