Upload CLI: on win32, use slashes, not os.sep

src/transformers/commands/user.py

 import os
+import sys
 from argparse import ArgumentParser
 from getpass import getpass
 from typing import List, Union
@@ -183,6 +184,9 @@ class UploadCommand(BaseUserCommand):
         else:
             raise ValueError("Not a valid file or directory: {}".format(local_path))
 
+        if sys.platform == "win32":
+            files = [(filepath, filename.replace(os.sep, "/")) for filepath, filename in files]
+
         if len(files) > UPLOAD_MAX_FILES:
             print(
                 "About to upload {} files to S3. This is probably wrong. Please filter files before uploading.".format(