fix bug in f4f8bf6ee which reintroduced the memory leak

kwant/solvers/sparse.py

@@ -15,11 +15,8 @@ from kwant import physics, system
 # reached all of our users.
 def del_for_umfpackcontext(self):
     self.free()
-try:
-    if not hasattr(spl.dsolve.umfpack.UmfpackContext, '__del__'):
-        spl.dsolve.umfpack.UmfpackContext.__del__ = del_for_umfpackcontext
-except:
-    pass
+if not hasattr(umfpack.UmfpackContext, '__del__'):
+    umfpack.UmfpackContext.__del__ = del_for_umfpackcontext
 del del_for_umfpackcontext
 
 def factorized(A, piv_tol=1.0, sym_piv_tol=1.0):

kwant/solvers/tests/test_sparse.py

@@ -280,4 +280,7 @@ def test_very_singular_leads():
     sys.attach_lead(right_lead)
     fsys = sys.finalized()
     result = solve(fsys)
-    assert result[1] == [0, 2]
\ No newline at end of file
+    assert result[1] == [0, 2]
+
+def test_umfpack_del():
+    assert hasattr(kwant.solvers.sparse.umfpack.UmfpackContext, '__del__')