The new packaging module should not use the locale encoding

Lib/packaging/metadata.py

 """Implementation of the Metadata for Python packages PEPs.
 
 Supports all metadata formats (1.0, 1.1, 1.2).
 """
 
+import codecs
 import re
 import logging
 
 from io import StringIO
 from email import message_from_file
 from packaging import logger
 from packaging.markers import interpret
 from packaging.version import (is_valid_predicate, is_valid_version,
                                is_valid_versions)
 from packaging.errors import (MetadataMissingError,
@@ skipping 307 matching lines @@
                     values = [tuple(value.split(',')) for value in values]
                 self.set(field, values)
             else:
                 # single line
                 value = msg[field]
                 if value is not None and value != 'UNKNOWN':
                     self.set(field, value)
 
     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)
             if field in _ELEMENTSFIELD:
                 self._write_field(fileobject, field, ','.join(values))
                 continue
             if field not in _LISTFIELDS:
                 if field == 'Description':
                     values = values.replace('\n', '\n       |')
                 values = [values]
@@ skipping 195 matching lines @@
 
     def __iter__(self):
         for key in self.keys():
             yield key
 
     def values(self):
         return [self[key] for key in list(self.keys())]
 
     def items(self):
         return [(key, self[key]) for key in list(self.keys())]