Argument Clinic: Use METH_FASTCALL for boring positionals

Argument Clinic: Use METH_FASTCALL for boring positionals

Issue #29286. Use METH_FASTCALL calling convention instead of METH_VARARGS to
parse "boring" position arguments. METH_FASTCALL is faster since it avoids the
creation of a temporary tuple to pass positional arguments.

Replace PyArg_UnpackTuple() with _PyArg_UnpackStack()+_PyArg_NoStackKeywords().