Remove iso8601 dependency from simple metadata api

Use builtin datetime instead of external iso6801 for simple
datetime string parsing. Also see
#1065

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

tuf/api/metadata.py

@@ -22,7 +22,6 @@
 from securesystemslib.storage import StorageBackendInterface
 from securesystemslib.keys import create_signature, verify_signature
 
-import iso8601
 import tuf.formats
 import tuf.exceptions
 
@@ -295,8 +294,9 @@ def from_dict(cls, signed_dict: JsonDict) -> 'Signed':
  # 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 'to_dict'.
- signed_dict['expires'] = iso8601.parse_date(
- signed_dict['expires']).replace(tzinfo=None)
+ signed_dict['expires'] = datetime.strptime(
+ signed_dict['expires'],
+ "%Y-%m-%dT%H:%M:%SZ").replace(tzinfo=None)
  # NOTE: We write the converted 'expires' back into 'signed_dict' above
  # so that we can pass it to the constructor as '**signed_dict' below,
  # along with other fields that belong to Signed subclasses.