find_sources: fix lint (#12403)

flake8-bugbear released a new version with new checks.
SourceFinder is small (and we don't have many of them), so this isn't a
particularly big deal.

mypy/find_sources.py

@@ -160,7 +160,7 @@ def crawl_up(self, path: str) -> Tuple[str, str]:
     def crawl_up_dir(self, dir: str) -> Tuple[str, str]:
         return self._crawl_up_helper(dir) or ("", dir)
 
-    @functools.lru_cache()
+    @functools.lru_cache()  # noqa: B019
     def _crawl_up_helper(self, dir: str) -> Optional[Tuple[str, str]]:
         """Given a directory, maybe returns module and base directory.