CObject to PyCapsule.

 Added checks for integer overflows, contributed by Google. Some are
 only available if asserts are left in the code, in cases where they
 can't be triggered from Python code.

 http://mail.python.org/pipermail/python-dev/2008-June/079988.html

Python 2.6 should stick with PyString_* in its codebase.  The PyBytes_* names
in the spirit of 3.0 are available via a #define only.  See the email thread.

function can be called recursively.
This was discussed in issue1020188.

In python codebase, all occurrences of Py_[X]DECREF(xxx->yyy) are suspect,
except when they appear in tp_new or tp_dealloc functions, or when
the member cannot be of a user-defined class.
Note that tp_init is not safe.

I do have a (crashing) example for every changed line.
Is it worth adding them to the test suite?

Example:

class SpecialStr(str):
    def __del__(self):
        s.close()

import cStringIO
s = cStringIO.StringIO(SpecialStr("text"))
s.close() # Segfault

Fixes #1730114.

backwards compatibility. Add Py_Refcnt, Py_Type, Py_Size, and
PyVarObject_HEAD_INIT.