Add version check for test (#1144)

* Add version check for test

* commit

hvplot/tests/testoperations.py

@@ -4,6 +4,7 @@
 from parameterized import parameterized
 
 import colorcet as cc
+import holoviews as hv
 import hvplot.pandas  # noqa
 import numpy as np
 import pandas as pd
@@ -12,6 +13,7 @@
 from holoviews.element import Image, QuadMesh, ImageStack
 from holoviews.element.comparison import ComparisonTestCase
 from hvplot.converter import HoloViewsConverter
+from packaging.version import Version
 
 
 class TestDatashader(ComparisonTestCase):
@@ -196,6 +198,8 @@ def test_datashade_rescale_discrete_levels_default_True(self):
         assert actual is expected
 
     def test_rasterize_by(self):
+        if Version(hv.__version__) < Version('1.18.0a1'):
+            raise SkipTest('hv.ImageStack introduced after 1.18.0a1')
         expected = 'category'
         plot = self.df.hvplot(x='x', y='y', by=expected, rasterize=True, dynamic=False)
         assert isinstance(plot, ImageStack)