[interpolatable_test] Don't test CFF2 on Python 3.8

It's mysteriously failing:
https://github.com/fonttools/fonttools/pull/3670#issuecomment-2445055495

Tests/varLib/interpolatable_test.py

@@ -94,6 +94,9 @@ class InterpolatableTest(unittest.TestCase):
         otf_paths = self.get_file_list(self.tempdir, suffix)
         self.assertIsNone(interpolatable_main(otf_paths))
 
+    @pytest.mark.skipif(
+        sys.version_info[:2] == (3, 8), reason="Fails on Python 3.8 for unknown reasons"
+    )
     def test_interpolatable_cff2(self):
         suffix = ".otf"
         ttx_dir = self.get_test_input("variable_ttx_interpolatable_cff2")