Add tkwantoperator to path in test script

tkwantoperator/tests/test_energy_operators.py

@@ -7,6 +7,10 @@
 # the file AUTHORS.rst at the top-level directory of this distribution and at
 # http://kwant-project.org/authors.
 
+import sys
+import os
+
+sys.path.insert(0, os.path.abspath('../tkwantoperator'))
 
 import numpy as np
 from numpy.testing import assert_allclose