Restore python 3.7 compatibile usage of functools.lru_cache (#480)

Author: francesco-ballarin

ffcx/element_interface.py

@@ -22,7 +22,7 @@
 import functools
 
 
-@functools.lru_cache
+@functools.lru_cache()
 def create_basix_element(family_type, cell_type, degree, variant_info, discontinuous):
     """Create a basix element."""
     return basix.create_element(family_type, cell_type, degree, *variant_info, discontinuous)