test: skip flaky test TestPlotLossesMethodNum on Windows

The test TestPlotLossesMethodNum showed to be flaky on Windows, see https://github.com/bancaditalia/black-it/actions/runs/5952400728/job/16144300857?pr=45#step:6:2581

Error output:

______________ TestPlotLossesMethodNum.test_run_by_method_num[3] ______________

self = <tests.test_plot.test_plot_results.TestPlotLossesMethodNum object at 0x000002399D891F30>
method_num = 3

    @pytest.mark.parametrize("method_num", list(range(5)))
    def test_run_by_method_num(
        self, method_num: int
    ) -> None:  # pylint: disable=arguments-differ
        """Run the test for all method numbers."""
        self.expected_image = (
            PLOT_DIR / f"plot_losses_method_num_{method_num}-expected.png"
        )
        self.args = [self.saving_folder, method_num]
>       super().run()

tests\test_plot\test_plot_results.py:86:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
tests\test_plot\base.py:43: in run
    self.plotting_function.__func__(*self.args)  # type: ignore
black_it\plot\plot_results.py:195: in plot_losses_method_num
    g = sns.pairplot(
.tox\py-nb\lib\site-packages\seaborn\axisgrid.py:2114: in pairplot
    grid = PairGrid(data, vars=vars, x_vars=x_vars, y_vars=y_vars, hue=hue,
.tox\py-nb\lib\site-packages\seaborn\axisgrid.py:1274: in __init__
    fig = plt.figure(figsize=figsize)
.tox\py-nb\lib\site-packages\matplotlib\_api\deprecation.py:454: in wrapper
    return func(*args, **kwargs)
.tox\py-nb\lib\site-packages\matplotlib\pyplot.py:840: in figure
    manager = new_figure_manager(
.tox\py-nb\lib\site-packages\matplotlib\pyplot.py:384: in new_figure_manager
    return _get_backend_mod().new_figure_manager(*args, **kwargs)
.tox\py-nb\lib\site-packages\matplotlib\backend_bases.py:3574: in new_figure_manager
    return cls.new_figure_manager_given_figure(num, fig)
.tox\py-nb\lib\site-packages\matplotlib\backend_bases.py:3579: in new_figure_manager_given_figure
    return cls.FigureCanvas.new_manager(figure, num)
.tox\py-nb\lib\site-packages\matplotlib\backend_bases.py:1742: in new_manager
    return cls.manager_class.create_with_canvas(cls, figure, num)
.tox\py-nb\lib\site-packages\matplotlib\backends\_backend_tk.py:483: in create_with_canvas
    window = tk.Tk(className="matplotlib")
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <tkinter.Tk object .>, screenName = None, baseName = 'pytest.EXE'
className = 'matplotlib', useTk = True, sync = False, use = None

    def __init__(self, screenName=None, baseName=None, className='Tk',
                 useTk=True, sync=False, use=None):
        """Return a new top level widget on screen SCREENNAME. A new Tcl interpreter will
        be created. BASENAME will be used for the identification of the profile file (see
        readprofile).
        It is constructed from sys.argv[0] without extensions if None is given. CLASSNAME
        is the name of the widget class."""
        self.master = None
        self.children = {}
        self._tkloaded = False
        # to avoid recursions in the getattr code in case of failure, we
        # ensure that self.tk is always _something_.
        self.tk = None
        if baseName is None:
            import os
            baseName = os.path.basename(sys.argv[0])
            baseName, ext = os.path.splitext(baseName)
            if ext not in ('.py', '.pyc'):
                baseName = baseName + ext
        interactive = False
>       self.tk = _tkinter.create(screenName, baseName, className, interactive, wantobjects, useTk, sync, use)
E       _tkinter.TclError: invalid command name "tcl_findLibrary"

C:\hostedtoolcache\windows\Python\3.10.11\x64\lib\tkinter\__init__.py:2299: TclError

Author: marcofavorito

tests/test_plot/test_plot_results.py

@@ -31,6 +31,7 @@
 )
 from tests.conftest import EXAMPLE_SAVING_FOLDER, PLOT_DIR
 from tests.test_plot.base import BasePlotResultsTest
+from tests.utils.base import skip_on_windows
 
 
 class TestPlotConvergence(
@@ -60,6 +61,7 @@ class TestPlotSampling(BasePlotResultsTest):  # pylint: disable=too-few-public-m
     expected_image = PLOT_DIR / "plot_sampling-expected.png"
 
 
+@skip_on_windows
 class TestPlotLossesMethodNum(
     BasePlotResultsTest
 ):  # pylint: disable=too-few-public-methods