Add a new public PyObject_CallNoArgs() function to the C API: call a
callable Python object without any arguments.

It is the most efficient way to call a callback without any argument.
On x86-64, for example, PyObject_CallFunctionObjArgs(func, NULL)
allocates 960 bytes on the stack per call, whereas
PyObject_CallNoArgs(func) only allocates 624 bytes per call.

It is excluded from stable ABI 3.8.

Replace private _PyObject_CallNoArg() with public
PyObject_CallNoArgs() in C extensions: _asyncio, _datetime,
_elementtree, _pickle, _tkinter and readline.

* Incref heap-allocated types in PyObject_Init
* Add documentation and porting notes to What's New

Fix also return type for few other functions (clear, releasebuffer).

Based on the investigation by Xiang Zhang.

when pass a string larger than 2 GiB.

Decrease memory requirements for Tcl's bigmem tests.

* Remove Setup.config
* Always define WITH_THREAD for compatibility.

There was few cases of using literal 0 instead of NULL in the context of
pointers.  While this was a legitimate C code, using NULL rather than 0 makes
the code clearer.