BUG: mypy: fix a typing error

This just adds `# type: ignore` to the line in
`scipy/_lib/_uarray/_backend.py` to ignore the
syntax error detected by `mypy`. This error seems
to be present only on Python 3.9.0a5 with mypy 0.770.

scipy/_lib/_uarray/_backend.py

@@ -93,7 +93,7 @@ def generate_multimethod(
     argument_extractor: ArgumentExtractorType,
     argument_replacer: ArgumentReplacerType,
     domain: str,
-    default: typing.Optional[typing.Callable] = None,
+    default: typing.Optional[typing.Callable] = None,  # type: ignore
 ):
     """
     Generates a multimethod.