Fix statsmodels FutureWarning for Yule-Walker estimation

[tsbinns]

pip-pre test runs failing due to a statsmodels FutureWarning (see e.g., https://github.com/mne-tools/mne-python/actions/runs/31015622468/job/92338994830?pr=13971):

mne/time_frequency/tests/test_ar.py:26: in test_yule_walker
    sm_rho, sm_sigma = sm_yw(d, order=2)
                       ^^^^^^^^^^^^^^^^^
/opt/hostedtoolcache/Python/3.14.6/x64/lib/python3.14/site-packages/statsmodels/regression/linear_model.py:1618: in yule_walker
    warnings.warn(
E   FutureWarning: yule_walker currently returns a plain tuple whose length depends on the inv argument. In release 0.16 or after July 2028, whichever is later, the default behavior will switch to always returning a YuleWalkerResult NamedTuple. Set use_namedtuple=True to switch now, or use_namedtuple=False to keep the current behavior and silence this warning.

These changes retain the current behaviour and silence the warning.

[tsbinns]

Maybe this can be dropped as well? https://github.com/tsbinns/mne-python/blob/670e836eafb30ddf933fe563783f3a7f98b94a8a/mne/conftest.py#L243-L251

[tsbinns]

No, still a warning for that which needs to be caught on pip-pre jobs

[larsoner]

Thanks @tsbinns !