Metadata API: Refactor strptime call

This makes flake8-datetimez happier but has no effect on the result:

    DTZ007 The use of `datetime.datetime.strptime()` without %z must be
    followed by `.replace(tzinfo=)` or `.astimezone()

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

Author: jku

tuf/api/_payload.py

@@ -181,8 +181,9 @@ def _common_fields_from_dict(
         # Convert 'expires' TUF metadata string to a datetime object, which is
         # what the constructor expects and what we store. The inverse operation
         # is implemented in '_common_fields_to_dict'.
-        expires = datetime.strptime(expires_str, "%Y-%m-%dT%H:%M:%SZ")
-        expires = expires.replace(tzinfo=timezone.utc)
+        expires = datetime.strptime(expires_str, "%Y-%m-%dT%H:%M:%SZ").replace(
+            tzinfo=timezone.utc
+        )
 
         return version, spec_version, expires