blender: verify expected Python version during build

Upstream builds and tests with a specific Python version. Further, the
Blender addon ecosystem generally expects that version to be used, so
ensure that the correct version is used when upstream changes. Python
often makes backwards-incompatible changes in “minor” releases.

To quote upstream source code:

> Blender only supports a single Python version at the moment.

Perform the check during preConfigure since we can check it then and so
it avoids wasting time configuring+building and then failing.

pkgs/applications/misc/blender/default.nix

@@ -198,6 +198,17 @@ stdenv.mkDerivation (finalAttrs: {
       "-DWITH_CYCLES_DEVICE_OPTIX=ON"
     ];
 
+  preConfigure = ''
+    (
+      expected_python_version=$(grep -E --only-matching 'set\(_PYTHON_VERSION_SUPPORTED [0-9.]+\)' build_files/cmake/Modules/FindPythonLibsUnix.cmake | grep -E --only-matching '[0-9.]+')
+      actual_python_version=$(python -c 'import sys; print(".".join(map(str, sys.version_info[0:2])))')
+      if ! [[ "$actual_python_version" = "$expected_python_version" ]]; then
+        echo "wrong Python version, expected '$expected_python_version', got '$actual_python_version'" >&2
+        exit 1
+      fi
+    )
+  '';
+
   nativeBuildInputs =
     [
       cmake