satpy/tests/scene_tests/test_resampling.py is using called_once in assertions rather than assert_called_once, causing test failures on Python 3.12

[ArrayBolt3]

Describe the bug
satpy tests fail with Python 3.12 due to the use of a method "called_once" that is not a valid assertion method. See python/cpython#100690

To Reproduce
Run the satpy tests using Python 3.12.

Expected behavior
No tests should fail.

Actual results

=================================== FAILURES ===================================
_____________ TestSceneResampling.test_resample_reduce_data_toggle _____________

self = <satpy.tests.scene_tests.test_resampling.TestSceneResampling object at 0x7fe3f80d82f0>
rs = <MagicMock name='resample_dataset' id='140615976571808'>

    @mock.patch('satpy.scene.resample_dataset')
    def test_resample_reduce_data_toggle(self, rs):
        """Test that the Scene can be reduced or not reduced during resampling."""
        from pyresample.geometry import AreaDefinition
    
        rs.side_effect = self._fake_resample_dataset_force_20x20
        proj_str = ('+proj=lcc +datum=WGS84 +ellps=WGS84 '
                    '+lon_0=-95. +lat_0=25 +lat_1=25 +units=m +no_defs')
        target_area = AreaDefinition('test', 'test', 'test', proj_str, 4, 4, (-1000., -1500., 1000., 1500.))
        area_def = AreaDefinition('test', 'test', 'test', proj_str, 5, 5, (-1000., -1500., 1000., 1500.))
        area_def.get_area_slices = mock.MagicMock()
        get_area_slices = area_def.get_area_slices
        get_area_slices.return_value = (slice(0, 3, None), slice(0, 3, None))
        area_def_big = AreaDefinition('test', 'test', 'test', proj_str, 10, 10, (-1000., -1500., 1000., 1500.))
        area_def_big.get_area_slices = mock.MagicMock()
        get_area_slices_big = area_def_big.get_area_slices
        get_area_slices_big.return_value = (slice(0, 6, None), slice(0, 6, None))
    
        # Test that data reduction can be disabled
        scene = Scene(filenames=['fake1_1.txt'], reader='fake1')
        scene.load(['comp19'])
        scene['comp19'].attrs['area'] = area_def
        scene['comp19_big'] = xr.DataArray(
            da.zeros((10, 10)), dims=('y', 'x'),
            attrs=scene['comp19'].attrs.copy())
        scene['comp19_big'].attrs['area'] = area_def_big
        scene['comp19_copy'] = scene['comp19'].copy()
        orig_slice_data = scene._slice_data
        # we force the below order of processing to test that success isn't
        # based on data of the same resolution being processed together
        test_order = [
            make_cid(**scene['comp19'].attrs),
            make_cid(**scene['comp19_big'].attrs),
            make_cid(**scene['comp19_copy'].attrs),
        ]
        with mock.patch('satpy.scene.Scene._slice_data') as slice_data, \
                mock.patch('satpy.dataset.dataset_walker') as ds_walker:
            ds_walker.return_value = test_order
            slice_data.side_effect = orig_slice_data
            scene.resample(target_area, reduce_data=False)
            assert not slice_data.called
            assert not get_area_slices.called
            scene.resample(target_area)
>           assert slice_data.called_once

satpy/tests/scene_tests/test_resampling.py:322: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <MagicMock name='_slice_data' id='140615696334768'>, name = 'called_once'

    def __getattr__(self, name):
        if name in {'_mock_methods', '_mock_unsafe'}:
            raise AttributeError(name)
        elif self._mock_methods is not None:
            if name not in self._mock_methods or name in _all_magics:
                raise AttributeError("Mock object has no attribute %r" % name)
        elif _is_magic(name):
            raise AttributeError(name)
        if not self._mock_unsafe and (not self._mock_methods or name not in self._mock_methods):
            if name.startswith(('assert', 'assret', 'asert', 'aseert', 'assrt')) or name in _ATTRIB_DENY_LIST:
>               raise AttributeError(
                    f"{name!r} is not a valid assertion. Use a spec "
                    f"for the mock if {name!r} is meant to be an attribute.")
E               AttributeError: 'called_once' is not a valid assertion. Use a spec for the mock if 'called_once' is meant to be an attribute.

/usr/lib/python3.12/unittest/mock.py:663: AttributeError

Environment Info:

• OS: Ubuntu 24.04 Development Version
• Satpy Version: 0.44.0
• PyResample Version: 1.27.1

Additional context
This bug can be resolved with the following patch (at least this is how I'm patching it in Ubuntu currently):

--- a/satpy/tests/scene_tests/test_resampling.py
+++ b/satpy/tests/scene_tests/test_resampling.py
@@ -319,14 +319,14 @@ class TestSceneResampling:
             assert not slice_data.called
             assert not get_area_slices.called
             scene.resample(target_area)
-            assert slice_data.called_once
-            assert get_area_slices.called_once
+            assert slice_data.assert_called_once
+            assert get_area_slices.assert_called_once
             scene.resample(target_area, reduce_data=True)
             # 2 times for each dataset
             # once for default (reduce_data=True)
             # once for kwarg forced to `True`
             assert slice_data.call_count == 2 * 3 
-            assert get_area_slices.called_once
+            assert get_area_slices.assert_called_once
 
     def test_resample_ancillary(self):
         """Test that the Scene reducing data does not affect final output."""

[mraspaud]

@ArrayBolt3 thanks for reporting this issue! Feel free to submit a pull request for it.
I think though that
slice_data.assert_called_once() should be use instead of
assert slice_data.assert_called_once, correct?

[ArrayBolt3]

You're right, I just didn't know what I was doing :P Technically the weird "assert assert" thing I did seems to work, but it is unnecessary and weird now that I'm looking closer at it.

I'll try and submit a (corrected) pull request sometime afternoon CST.