--- a/Lib/packaging/tests/support.py
+++ b/Lib/packaging/tests/support.py
@@ -146,7 +146,7 @@ class TempdirManager:
         """
         if isinstance(path, (list, tuple)):
             path = os.path.join(*path)
-        f = open(path, 'w')
+        f = open(path, 'w', encoding='utf8')
         try:
             f.write(content)
         finally: