Fix mypy

Author: Guido van Rossum

mypy/build.py

@@ -1961,7 +1961,7 @@ def valid_references(self) -> Set[str]:
 
         return valid_refs
 
-    def write_cache(self, graph) -> None:
+    def write_cache(self, graph: Graph) -> None:
         assert self.tree is not None, "Internal error: method must be called on parsed file only"
         if not self.path or self.options.cache_dir == os.devnull:
             return