The new packaging module should not use the locale encoding

Lib/packaging/metadata.py

--- a/Lib/packaging/metadata.py
+++ b/Lib/packaging/metadata.py
@@ -3,6 +3,7 @@
 Supports all metadata formats (1.0, 1.1, 1.2).
 """
 
+import codecs
 import re
 import logging
 
@@ -330,11 +331,16 @@ class Metadata:
 
     def write(self, filepath):
         """Write the metadata fields to filepath."""
-        with open(filepath, 'w') as fp:
+        with open(filepath, 'w', encoding='utf-8') as fp:
             self.write_file(fp)
 
     def write_file(self, fileobject):
         """Write the PKG-INFO format data to a file object."""
+        if not isinstance(fileobject, StringIO):
+            encoding = codecs.lookup(fileobject.encoding).name
+            if encoding != 'utf-8':
+                raise ValueError("Output file should be a UTF-8 text file, not %s encoding" % encoding)
+
         self._set_best_version()
         for field in _version2fieldlist(self['Metadata-Version']):
             values = self.get(field)