Add Python < 3.3 support for pyabc.i

pyabc.i for abstract base classes now supports versions of Python
prior to 3.3 by using the collection module for these older versions.
Python-3.3 and later continue to use the collections.abc module.
The -py3 option no longer has any effect on the %pythonabc feature.

Examples/test-suite/python_abstractbase.i

@@ -24,9 +24,3 @@ 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