Fix sphinx.testing.path.path.write_bytes bytes type (#12790)

Author: adamtheturtle

sphinx/testing/path.py

@@ -178,7 +178,7 @@ def read_bytes(self) -> builtins.bytes:
         with open(self, mode='rb') as f:
             return f.read()
 
-    def write_bytes(self, bytes: str, append: bool = False) -> None:
+    def write_bytes(self, bytes: bytes, append: bool = False) -> None:
         """
         Writes the given `bytes` to the file.