skip tests if optional packages are not found

tests/examples/test_djvu2png.py

@@ -1,11 +1,20 @@
 import os
 import subprocess
+from importlib.util import find_spec
 from tempfile import NamedTemporaryFile
+from unittest import SkipTest
 
 from tests.tools import EXAMPLES, IMAGES, TestCase
 
 
 class Djvu2PngTestCase(TestCase):
+    @classmethod
+    def setUpClass(cls):
+        if find_spec('numpy') is None:
+            raise SkipTest('Package numpy not found.')
+        if find_spec('cairocffi') is None or find_spec('pycairo') is None:
+            raise SkipTest('Cairo bindings not found.')
+
     def check(self, mode: str):
         with NamedTemporaryFile(suffix='.png') as outfile:
             subprocess.run(