Update autotest/gdrivers/vrtderived.py

Author: rouault

autotest/gdrivers/vrtderived.py

@@ -1266,6 +1266,7 @@ def handle(ecls, ecode, emsg):
 @pytest.mark.parametrize("fn", ["sum", "mul"])
 def test_vrt_pixelfn_constant_factor(tmp_vsimem, fn):
 
+    gdaltest.importorskip_gdal_array()
     np = pytest.importorskip("numpy")
     k = 7