Pandas deprecations

[kandersolar]

• [ ] Closes #xxxx
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• [ ] Tests added
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A couple pandas deprecation/future warnings have popped up. From: https://github.com/pvlib/pvlib-python/actions/runs/19341083174/job/55329373615

=============================== warnings summary ===============================
tests/iotools/test_sodapro.py:24
  /home/runner/work/pvlib-python/pvlib-python/tests/iotools/test_sodapro.py:24: FutureWarning: 'M' is deprecated and will be removed in a future version, please use 'ME' instead.
    index_monthly = pd.date_range('2020-01-01', periods=4, freq='1M')

tests/iotools/test_acis.py::test_get_acis_station_data
tests/iotools/test_acis.py::test_get_acis_station_data
  /home/runner/work/pvlib-python/pvlib-python/pvlib/iotools/acis.py:416: FutureWarning: Downcasting behavior in `replace` is deprecated and will be removed in a future version. To retain the old behavior, explicitly call `result.infer_objects(copy=False)`. To opt-in to the future behavior, set `pd.set_option('future.no_silent_downcasting', True)`
    df = df.replace("M", np.nan)

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html

[kandersolar]

test failures are unrelated (meteonorm issue)