test_dhparser.py: python3 in call instead of python (compatibility with older systems)

test/test_dhparser.py

@@ -45,9 +45,9 @@ class TestDHParserCommandLineTool:
 
     def test_dhparser(self):
         nulldevice = " >/dev/null" if platform.system() != "Windows" else " > NUL"
-        os.system('python ../scripts/dhparser.py testdata/neu ' + nulldevice)
-        os.system('python testdata/neu/tst_neu_grammar.py ' + nulldevice)
-        os.system('python testdata/neu/neuCompiler.py testdata/neu/example.dsl '
+        os.system('python3 ../scripts/dhparser.py testdata/neu ' + nulldevice)
+        os.system('python3 testdata/neu/tst_neu_grammar.py ' + nulldevice)
+        os.system('python3 testdata/neu/neuCompiler.py testdata/neu/example.dsl '
                   '>testdata/neu/example.xml')
         with open('testdata/neu/example.xml', 'r', encoding='utf-8') as f:
             xml = f.read()