XEP-0203 plugin allows timestamp without TZINFO

According to the spec:

The Time Zone Definition is mandatory

But this doesn't raise any warning in slixmpp:

from slixmpp import Message
import slixmpp.plugins.xep_0203
from slixmpp.xmlstream import register_stanza_plugin

register_stanza_plugin(Message, slixmpp.plugins.xep_0203.stanza.Delay)
m=Message()
m["delay"].set_stamp(datetime.datetime.now())
print(m)

<message xmlns="jabber:client">
  <delay xmlns="urn:xmpp:delay" stamp="2022-07-12T09:28:34.669984" />  <!-- where is the TZ info? -->
</message>

I did not investigate enough to know where to fix this, but I noticed it uses slixmpp.thirdparty.mini_dateutil which has this in its header:

# dateutil - Extensions to the standard python 2.3+ datetime module.

I wonder if it is still required with python 3+...