Revert "Make slots a tuple instead of a list"

This reverts commit 250391ae.

python/dune/perftool/ufl/execution.py

@@ -140,7 +140,7 @@ def split_expression(expr):
 
 class FiniteElement(ufl.FiniteElement):
 
-    __slots = ufl.FiniteElement.__slots__ + ("dirichlet_expression", "dirichlet_constraints")
+    __slots = ufl.FiniteElement.__slots__ + ["dirichlet_expression", "dirichlet_constraints"]
 
     def __new__(cls, *args, **kwargs):
         kwargs.pop('dirichlet_constraints', None)