[Python-Dev] Add Py_REPLACE and Py_XREPLACE macros

[Serhiy Storchaka]

The Py_CLEAR macros is used as safe alternative for following unsafe 
idiomatic code:

     Py_XDECREF(ptr);
     ptr = NULL;

But other unsafe idiomatic code is widely used in the sources:

     Py_XDECREF(ptr);
     ptr = new_value;

Every occurrence of such code is potential bug for same reasons as for 
Py_CLEAR.

It was offered [1] to introduce new macros Py_REPLACE and Py_XREPLACE 
for safe replace with Py_DECREF and Py_XDECREF respectively. 
Automatically generated patch contains about 50 replaces [2].

[1] http://bugs.python.org/issue16447
[2] http://bugs.python.org/issue20440