Fix signature upload

     def post(self):
         """Accept PyPI package upload."""
         form = web.form(":action")
-        if form[":action"] != "file_upload":
+        if form[":action"] not in ("sig_upload", "file_upload"):
             raise web.BadRequest(f"Provided `:action={form[':action']}` not supported.")
         try:
             release_file = form.content.save(file_dir="/tmp")