Fix mypy checks (typing.Optional) (#545)

* typing.Optional

* flake

Author: mscroggs

ffcx/compiler.py

@@ -81,7 +81,7 @@ def _print_timing(stage: int, timing: float):
 
 def compile_ufl_objects(ufl_objects: typing.List[typing.Any],
                         object_names: typing.Dict = {},
-                        prefix: str = None,
+                        prefix: typing.Optional[str] = None,
                         options: typing.Dict = {},
                         visualise: bool = False):
     """Generate UFC code for a given UFL objects.

ffcx/element_interface.py

@@ -108,9 +108,9 @@ class QuadratureElement(basix.ufl_wrapper._BasixElementBase):
     _cellname: str
 
     def __init__(
-        self, cellname: str, value_shape: typing.Tuple[int, ...], scheme: str = None, degree: int = None,
-        points: basix.ufl_wrapper._nda_f64 = None, weights: basix.ufl_wrapper._nda_f64 = None,
-        mapname: str = "identity"
+        self, cellname: str, value_shape: typing.Tuple[int, ...], scheme: typing.Optional[str] = None,
+        degree: typing.Optional[int] = None, points: typing.Optional[basix.ufl_wrapper._nda_f64] = None,
+        weights: typing.Optional[basix.ufl_wrapper._nda_f64] = None, mapname: str = "identity"
     ):
         """Initialise the element."""
         if scheme is not None: