slot_sq_item() now calls _PyObject_FastCall() to avoid the creation of a
temporary tuple of 1 item to pass the 'item' argument to the slot function.

* Invert condition of test to avoid levels of indentation
* Remove useless Py_XDECREF(args) in the error block
* Replace Py_XDECREF(func) with Py_DECREF(func) in the error block: func cannot
  be NULL when reaching the error block

Issue #27128. The call_method() and call_maybe() functions of typeobject.c now
use fast call for empty format string to avoid the creation of a temporary
empty tuple.

Issue #27128. Move va_start/va_end around Py_VaBuildValue().

Issue #27128. Fix a reference leak if creating the tuple to pass positional
parameters fails.

Issue #27128. Fix a reference leak if creating the tuple to pass positional
parameters fails.

Issue #27128. Modify slot_sq_contains() and slot_tp_richcompare() to use fast
call to avoid a temporary tuple to pass a single positional parameter.

Issue #27128: arg=>args, kw=>kwargs.

Same change for PyEval_CallObjectWithKeywords().

Issue #27128, PyObject_CallFunction(), _PyObject_FastCall() and callmethod():
if the format string of parameters is empty, avoid the creation of an empty
tuple: call _PyObject_FastCall() without parameters.

Issue #27128.

Make callmethod() less weird: don't decrement func reference counter,
the caller is now responsible to do that.

Issue #27128.

Make call_function_tail() less weird: don't decrement args reference counter,
the caller is now responsible to do that. The caller now also checks if args is
NULL.

Issue #27128.

Issue #27128: Modify call_function_tail() to use _PyObject_FastCall() when args
is not a tuple to avoid the creation of a temporary tuple.

call_function_tail() is used by:

* PyObject_CallFunction()
* PyObject_CallMethod()
* _PyObject_CallMethodId()

Issue #27128: Modify PyEval_CallObjectWithKeywords() to use
_PyObject_FastCall() when args==NULL and kw==NULL. It avoids the creation of a
temporary empty tuple for positional arguments.

Issue #27128: Add _PyObject_FastCall(), a new calling convention avoiding a
temporary tuple to pass positional parameters in most cases, but create a
temporary tuple if needed (ex: for the tp_call slot).

The API is prepared to support keyword parameters, but the full implementation
will come later (_PyFunction_FastCall() doesn't support keyword parameters
yet).

Add also:

* _PyStack_AsTuple() helper function: convert a "stack" of parameters to
  a tuple.
* _PyCFunction_FastCall(): fast call implementation for C functions
* _PyFunction_FastCall(): fast call implementation for Python functions

Clang is smarter than GCC and emits a warning for dead code after a function
declared with __attribute__((__noreturn__)) (Py_FatalError).

PyModule_NewObject already sets md_dict to PyDict_New():

    m->md_dict = PyDict_New();

Patch by Eryk Sun and Emanuel Barry.

Also moved news item for #2466 to the correct place.

Patch by Robin Roth, reviewed by Serhiy Storchaka, comment wording
tweaked by me.

- Issue #25958: Support "anti-registration" of special methods from
  various ABCs, like __hash__, __iter__ or __len__.  All these (and
  several more) can be set to None in an implementation class and the
  behavior will be as if the method is not defined at all.
  (Previously, this mechanism existed only for __hash__, to make
  mutable classes unhashable.)  Code contributed by Andrew Barnert and
  Ivan Levkivskyi.

Issue #27558: Fix a SystemError in the implementation of "raise" statement.
In a brand new thread, raise a RuntimeError since there is no active
exception to reraise.

Patch written by Xiang Zhang.

Does not appear to be a problem anymore and I cannot figure
out why it was skipped in the first place.

enabled: ensure code object has a valid first line number.
Patch suggested by Ivan Levkivskyi.

The z variable is known to be a fresh number which cannot be shared, Py_SIZE()
can be used directly to negate the number.