add missing import

src/pymortests/pickling.py

@@ -7,6 +7,7 @@ from scipy.sparse import issparse
 from types import FunctionType, MethodType
 
 from pymor.core.base import BasicObject
+from pymor.core.config import config
 from pymor.core.pickle import dumps, loads, dumps_function, PicklingError
 from pymor.discretizers.builtin.grids.subgrid import SubGrid
 from pymor.operators.numpy import NumpyMatrixBasedOperator