dict.get() and dict.setdefault() now use AC

Issue #29311: dict.get() and dict.setdefault() methods now use Argument Clinic
to parse arguments. Their calling convention changes from METH_VARARGS to
METH_FASTCALL which avoids the creation of a temporary tuple.

The signature of docstrings is also enhanced. For example,
    get(...)
becomes:
    get(self, key, default=None, /)