test: add pytest.mark.xfail for flaky `scatter_with_layered_histogr…

…am` test

Struggling to find the source of the failure, the `mark` is my best guess but currently can't reproduce the following error:

```
___________________________________________ test_compound_chart_examples[False-scatter_with_layered_histogram.py-all_rows18-all_cols18] ___________________________________________
[gw2] win32 -- Python 3.8.19 C:\Users\*\AppData\Local\hatch\env\virtual\altair\CXM7NV9I\hatch-test.py3.8\Scripts\python.exe

filename = 'scatter_with_layered_histogram.py', all_rows = [2, 17], all_cols = [['gender'], ['__count']], to_reconstruct = False

    @pytest.mark.skipif(vf is None, reason="vegafusion not installed")
    # fmt: off
    @pytest.mark.parametrize("filename,all_rows,all_cols", [
        ("errorbars_with_std.py", [10, 10], [["upper_yield"], ["extent_yield"]]),
        ("candlestick_chart.py", [44, 44], [["low"], ["close"]]),
        ("co2_concentration.py", [713, 7, 7], [["first_date"], ["scaled_date"], ["end"]]),
        ("falkensee.py", [2, 38, 38], [["event"], ["population"], ["population"]]),
        ("heat_lane.py", [10, 10], [["bin_count_start"], ["y2"]]),
        ("histogram_responsive.py", [20, 20], [["__count"], ["__count"]]),
        ("histogram_with_a_global_mean_overlay.py", [9, 1], [["__count"], ["mean_IMDB_Rating"]]),
        ("horizon_graph.py", [20, 20], [["x"], ["ny"]]),
        ("interactive_cross_highlight.py", [64, 64, 13], [["__count"], ["__count"], ["Major_Genre"]]),
        ("interval_selection.py", [123, 123], [["price_start"], ["date"]]),
        ("layered_chart_with_dual_axis.py", [12, 12], [["month_date"], ["average_precipitation"]]),
        ("layered_heatmap_text.py", [9, 9], [["Cylinders"], ["mean_horsepower"]]),
        ("multiline_highlight.py", [560, 560], [["price"], ["date"]]),
        ("multiline_tooltip.py", [300, 300, 300, 0, 300], [["x"], ["y"], ["y"], ["x"], ["x"]]),
        ("pie_chart_with_labels.py", [6, 6], [["category"], ["value"]]),
        ("radial_chart.py", [6, 6], [["values"], ["values_start"]]),
        ("scatter_linked_table.py", [392, 14, 14, 14], [["Year"], ["Year"], ["Year"], ["Year"]]),
        ("scatter_marginal_hist.py", [34, 150, 27], [["__count"], ["species"], ["__count"]]),
        ("scatter_with_layered_histogram.py", [2, 17], [["gender"], ["__count"]]),
        ("scatter_with_minimap.py", [1461, 1461], [["date"], ["date"]]),
        ("scatter_with_rolling_mean.py", [1461, 1461], [["date"], ["rolling_mean"]]),
        ("seattle_weather_interactive.py", [1461, 5], [["date"], ["__count"]]),
        ("select_detail.py", [20, 1000], [["id"], ["x"]]),
        ("simple_scatter_with_errorbars.py", [5, 5], [["x"], ["upper_ymin"]]),
        ("stacked_bar_chart_with_text.py", [60, 60], [["site"], ["site"]]),
        ("us_employment.py", [120, 1, 2], [["month"], ["president"], ["president"]]),
        ("us_population_pyramid_over_time.py", [19, 38, 19], [["gender"], ["year"], ["gender"]]),
    ])
    # fmt: on
    @pytest.mark.parametrize("to_reconstruct", [True, False])
    def test_compound_chart_examples(filename, all_rows, all_cols, to_reconstruct):
        source = pkgutil.get_data(examples_methods_syntax.__name__, filename)
        chart = eval_block(source)
        if to_reconstruct:
            # When reconstructing a Chart, Altair uses different classes
            # then what might have been originally used. See
            # vega/vegafusion#354 for more info.
            chart = alt.Chart.from_dict(chart.to_dict())
        dfs = chart.transformed_data()

        if not to_reconstruct:
            # Only run assert statements if the chart is not reconstructed. Reason
            # is that for some charts, the original chart contained duplicated datasets
            # which disappear when reconstructing the chart.
            assert len(dfs) == len(all_rows)
            for df, rows, cols in zip(dfs, all_rows, all_cols):
>               assert len(df) == rows
E               assert 19 == 17
E                +  where 19 = len(    bin_step_5_age  bin_step_5_age_end gender  __count\n0             45.0                50.0      M      247\n1       ...       11\n17
   30.0                35.0      F        5\n18            70.0                75.0      F        1)

tests\test_transformed_data.py:132: AssertionError
```

tests/test_transformed_data.py

@@ -1,4 +1,5 @@
 import pkgutil
+import sys
 
 import pytest
 from vega_datasets import data
@@ -101,7 +102,16 @@ def test_primitive_chart_examples(filename, rows, cols, to_reconstruct):
     ("radial_chart.py", [6, 6], [["values"], ["values_start"]]),
     ("scatter_linked_table.py", [392, 14, 14, 14], [["Year"], ["Year"], ["Year"], ["Year"]]),
     ("scatter_marginal_hist.py", [34, 150, 27], [["__count"], ["species"], ["__count"]]),
-    ("scatter_with_layered_histogram.py", [2, 19], [["gender"], ["__count"]]),
+    pytest.param(
+        "scatter_with_layered_histogram.py",
+        [2, 19],
+        [["gender"], ["__count"]],
+        marks=pytest.mark.xfail(
+            sys.version_info <= (3, 9),
+            reason="Possibly `numpy` with unsupported python.\n"
+            "Very intermittent, but only affects `to_reconstruct=False`."
+        ),
+    ),
     ("scatter_with_minimap.py", [1461, 1461], [["date"], ["date"]]),
     ("scatter_with_rolling_mean.py", [1461, 1461], [["date"], ["rolling_mean"]]),
     ("seattle_weather_interactive.py", [1461, 5], [["date"], ["__count"]]),