nix: use default python version for nix dev environment (#5672)

This is a follow-up of #5671.

Author: imincik

flake.nix

@@ -21,44 +21,39 @@
 
         packages.grass = pkgs.callPackage ./package.nix { };
 
-        devShells.default =
-          let
-            pyPackages = pkgs.python312Packages;
-
-          in
-          pkgs.mkShell {
-            inputsFrom = [ self'.packages.grass ];
-
-            # additional packages
-            buildInputs =  with pyPackages; [
-                pytest
-              ];
-
-            LOCALE_ARCHIVE = "${pkgs.glibcLocales}/lib/locale/locale-archive";
-
-            shellHook = ''
-              function dev-help {
-                echo -e "\nWelcome to a GRASS development environment !"
-                echo "Build GRASS using following commands:"
-                echo
-                echo " 1.  ./configure --prefix=\$(pwd)/app"
-                echo " 2.  make -j$(nproc)"
-                echo " 3.  make install"
-                echo
-                echo "Run tests:"
-                echo
-                echo " 1.  export PYTHONPATH=\$(app/bin/grass --config python_path):\$PYTHONPATH"
-                echo " 2.  export LD_LIBRARY_PATH=\$(app/bin/grass --config path)/lib:\$LD_LIBRARY_PATH"
-                echo " 3.  pytest"
-                echo
-                echo "Note: run 'nix flake update' from time to time to update dependencies."
-                echo
-                echo "Run 'dev-help' to see this message again."
-              }
-
-              dev-help
-            '';
-          };
+        devShells.default = pkgs.mkShell {
+          inputsFrom = [ self'.packages.grass ];
+
+          # additional packages
+          buildInputs = with pkgs.python3Packages; [
+            pytest
+          ];
+
+          LOCALE_ARCHIVE = "${pkgs.glibcLocales}/lib/locale/locale-archive";
+
+          shellHook = ''
+            function dev-help {
+              echo -e "\nWelcome to a GRASS development environment !"
+              echo "Build GRASS using following commands:"
+              echo
+              echo " 1.  ./configure --prefix=\$(pwd)/app"
+              echo " 2.  make -j$(nproc)"
+              echo " 3.  make install"
+              echo
+              echo "Run tests:"
+              echo
+              echo " 1.  export PYTHONPATH=\$(app/bin/grass --config python_path):\$PYTHONPATH"
+              echo " 2.  export LD_LIBRARY_PATH=\$(app/bin/grass --config path)/lib:\$LD_LIBRARY_PATH"
+              echo " 3.  pytest"
+              echo
+              echo "Note: run 'nix flake update' from time to time to update dependencies."
+              echo
+              echo "Run 'dev-help' to see this message again."
+            }
+
+            dev-help
+          '';
+        };
       };
 
       flake = { };