do not emit unused warning for variables named unused*

Cython has its own warning for unused variables, which is enabled with `-Wextra`. However, the warning does not trigger if the variable is named `_` or if the variable name starts with `unused`.

cython_lint/cython_lint.py

@@ -256,6 +256,7 @@ def visit_funcdef(
             and _def[0] != func_name
             and _def[0] not in [i[0] for i in args]
             and not _def[0].startswith('_')
+            and not _def[0].startswith('unused')
             and _def[0] not in global_names
         ):