Run black over recent immutable sequence view tests

Now that we're running black on tests...

tests/test_immutable_sequence_view.py

@@ -3,6 +3,7 @@ import pytest
 from typing import Sequence
 from textual._immutable_sequence_view import ImmutableSequenceView
 
+
 def wrap(source: Sequence[int]) -> ImmutableSequenceView[int]:
     """Wrap a sequence of integers inside an immutable sequence view."""
     return ImmutableSequenceView[int](source)