tuf.api: fix loading expiration

Signed-off-by: Joshua Lock <jlock@vmware.com>

tuf/api/metadata.py

@@ -15,6 +15,7 @@
 
 # 3rd-party.
 from dateutil.relativedelta import relativedelta
+import iso8601
 from securesystemslib.formats import encode_canonical
 from securesystemslib.keys import create_signature, verify_signature
 from securesystemslib.util import load_json_file
@@ -51,7 +52,8 @@ def read_from_json(self, filename: str) -> None:
  self.signatures = signable['signatures']
  self.signed = signable['signed']
 
- self.expiration = datetime.strptime(signed['expiration'], '%b %d %Y %I:%M%p')
+ # TODO: replace with dateutil.parser.parse?
+ self.expiration = iso8601.parse_date(self.signed['expires'])
  self.version = self.signed['version']
 
  fn, fn_ver = _strip_version_number(filename, True)