Metadata API: Set default expires to utcnow()

This means the metadata is by default expired: this seems like a fine
default since we only allow a default value for practical reasons (not
allowing it would mean backwards incompatible API change).

Signed-off-by: Jussi Kukkonen <jkukkonen@vmware.com>

Author: Jussi Kukkonen

tuf/api/metadata.py

@@ -32,7 +32,7 @@
 import io
 import logging
 import tempfile
-from datetime import datetime, timedelta
+from datetime import datetime
 from typing import (
     IO,
     Any,
@@ -500,7 +500,7 @@ def __init__(
 
         self.spec_version = spec_version
 
-        self.expires = expires or datetime.utcnow() + timedelta(days=1)
+        self.expires = expires or datetime.utcnow()
 
         if version is None:
             version = 1