Add METH_FASTCALL calling convention

Issue #27810: Add a new calling convention for C functions:

    PyObject* func(PyObject *self, PyObject **args,
                   Py_ssize_t nargs, PyObject *kwnames);

Where args is a C array of positional arguments followed by values of keyword
arguments. nargs is the number of positional arguments, kwnames are keys of
keyword arguments. kwnames can be NULL.