If only offsetof() is needed: include stddef.h instead.

When structmember.h is used, add a comment explaining that
PyMemberDef is used.

Update _asyncio, _bz2, _csv, _curses, _datetime,
_io, _operator, _pickle, _queue, blake2,
multibytecodec and overlapped C extension modules
to use PyModule_AddType().

The bulk of this patch was generated automatically with:

    for name in \
        PyObject_Vectorcall \
        Py_TPFLAGS_HAVE_VECTORCALL \
        PyObject_VectorcallMethod \
        PyVectorcall_Function \
        PyObject_CallOneArg \
        PyObject_CallMethodNoArgs \
        PyObject_CallMethodOneArg \
    ;
    do
        echo $name
        git grep -lwz _$name | xargs -0 sed -i "s/\b_$name\b/$name/g"
    done

    old=_PyObject_FastCallDict
    new=PyObject_VectorcallDict
    git grep -lwz $old | xargs -0 sed -i "s/\b$old\b/$new/g"

and then cleaned up:

- Revert changes to in docs & news
- Revert changes to backcompat defines in headers
- Nudge misaligned comments

Replace direct access to PyObject.ob_type with Py_TYPE().

If an exception is raised and PyInit__multibytecodec() returns NULL,
Python reports properly the exception to the user. There is no need
to crash Python with Py_FatalError().

In ArgumentClinic, value "NULL" should now be used only for unrepresentable default values
(like in the optional third parameter of getattr). "None" should be used if None is accepted
as argument and passing None has the same effect as not passing the argument at all.