Use python3 as the interpreter instead of python (ie v2) more places.

This fixes lintian warning "unusual-interpreter".

Part of the Debian edition of FreeCAD since 2023.

src/Mod/Mesh/App/MeshTestsApp.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 
 #  Copyright (c) 2007 Jürgen Riegel <juergen.riegel@web.de>

src/Mod/Robot/KukaExporter.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 
 # Kuka export lib (c) Jürgen Riegel 2009 LGPL 2.1 or higher

src/Mod/Robot/MovieTool.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/python3
 import os,sys,string
 import FreeCAD,FreeCADGui,Robot,RobotGui