Replace:
    PyObject_CallFunctionObjArgs(callable, NULL)
with:
    _PyObject_CallNoArg(callable)

Replace:
    PyObject_CallObject(callable, NULL)
with:
    _PyObject_CallNoArg(callable)

Remove most indentation to move code at the left.

again.  But they still are deprecated and will be disabled in 3.7.

Issue #28838: Rename parameters of the "calls" functions of the Python C API.

* Rename 'callable_object' and 'func' to 'callable': any Python callable object
  is accepted, not only Python functions
* Rename 'method' and 'nameid' to 'name' (method name)
* Rename 'o' to 'obj'
* Move, fix and update documentation of PyObject_CallXXX() functions
  in abstract.h
* Update also the documentaton of the C API (update parameter names)

The fix for catch_warnings() is the issue #28835 (not the issue #28089).

Issue #28089: Fix a regression introduced in warnings.catch_warnings(): call
warnings.showwarning() if it was overriden inside the context manager.

Modify type_setattro() to call directly _PyObject_GenericSetAttrWithDict()
instead of PyObject_GenericSetAttr().

PyObject_GenericSetAttr() is a thin wrapper to
_PyObject_GenericSetAttrWithDict().

Replace 0 with (0) to ignore a compiler warning about dead code on
"((int)(SEM_VALUE_MAX) < 0)": SEM_VALUE_MAX is not negative on Linux.

Replace C if() with precompiler #if to fix a warning on dead code when using
clang.

Don't declare dead code when the code is declared with Clang.

Replace
   _PyObject_CallArg1(func, arg)
with
   PyObject_CallFunctionObjArgs(func, arg, NULL)

Using the _PyObject_CallArg1() macro increases the usage of the C stack, which
was unexpected and unwanted. PyObject_CallFunctionObjArgs() doesn't have this
issue.

Handling zero-argument super() in __init_subclass__ and
__set_name__ involved moving __class__ initialisation to
type.__new__. This requires cooperation from custom
metaclasses to ensure that the new __classcell__ entry
is passed along appropriately.

The initial implementation of that change resulted in abruptly
broken zero-argument super() support in metaclasses that didn't
adhere to the new requirements (such as Django's metaclass for
Model definitions).

The updated approach adopted here instead emits a deprecation
warning for those cases, and makes them work the same way they
did in Python 3.5.

This patch also improves the related class machinery documentation
to cover these details and to include more reader-friendly
cross-references and index entries.

Issue #28858: The change b9c9691c72c5 introduced a regression. It seems like
_PyObject_CallArg1() uses more stack memory than
PyObject_CallFunctionObjArgs().