Make gen_exports not use carriage returns on Windows

src/trio/_tools/gen_exports.py

@@ -303,7 +303,7 @@ def process(files: Iterable[File], *, do_test: bool) -> None:
             print("Generated sources are up to date.")
     else:
         for new_path, new_source in new_files.items():
-            with open(new_path, "w", encoding="utf-8") as f:
+            with open(new_path, "w", encoding="utf-8", newline="\n") as f:
                 f.write(new_source)
         print("Regenerated sources successfully.")
         if not matches_disk: