don't use nested namespaces if not at least C++17

test/test_lowlevel.py

@@ -17,6 +17,9 @@ def setup_class(cls):
         cls.datatypes = cppyy.load_reflection_info(cls.test_dct)
         cls.N = cppyy.gbl.N
 
+        at_least_17 = 201402 < cppyy.gbl.gInterpreter.ProcessLine("__cplusplus;")
+        cls.has_nested_namespace = at_least_17
+
     def test00_import_all(self):
         """Validity of `from cppyy.ll import *`"""
 
@@ -493,6 +496,9 @@ def test14_templated_arrays(self):
     def test15_templated_arrays_gmpxx(self):
         """Use of gmpxx array types in templates"""
 
+        if not self.has_nested_namespace:
+            return
+
         import cppyy
 
         try: