Python: Add false negative test case.

python/ql/test/query-tests/Variables/unused/type_annotation_fp.py

@@ -9,3 +9,8 @@ def type_annotation(x):
     else:
         foo : float
         do_other_stuff_with(foo)
+
+def type_annotation_fn():
+    # False negative: the value of `bar` is never used, but this is masked by the presence of the type annotation.
+    bar = 5
+    bar : int