Only test python:abc feature when passing -py3 to swig

Examples/test-suite/python/python_abstractbase_runme3.py

@@ -6,6 +6,10 @@ from collections import *
 if is_python_builtin():
     exit(0)
 
+# Python abc is only turned on when -py3 option is passed to SWIG
+if not is_swig_py3:
+    exit(0)
+
 assert issubclass(Mapii, MutableMapping)
 assert issubclass(Multimapii, MutableMapping)
 assert issubclass(IntSet, MutableSet)

Examples/test-suite/python_abstractbase.i

@@ -24,3 +24,9 @@ bool is_python_builtin() { return true; }
 bool is_python_builtin() { return false; }
 #endif
 %}
+
+#ifdef SWIGPYTHON_PY3 // set when using -py3
+#define is_swig_py3 1
+#else
+#define is_swig_py3 0
+#endif