Keep jacobian_inames as a tuple

The old Python2 vs. Python3 problem: non-deterministic hash produces the best bugs!!!

python/dune/perftool/sumfact/symbolic.py

@@ -184,10 +184,10 @@ class SumfactKernel(SumfactKernelBase, ImmutableRecord, prim.Variable):
     @property
     def within_inames(self):
         if self.trial_element is None:
-            return frozenset()
+            return ()
         else:
             from dune.perftool.sumfact.basis import lfs_inames
-            return frozenset(lfs_inames(self.trial_element, self.restriction))
+            return lfs_inames(self.trial_element, self.restriction)
 
     def vec_index(self, sf):
         """ Map an unvectorized sumfact kernel object to its position