chore: update client.py for python 3.12 utcnow deprecation

client.py

@@ -3,7 +3,8 @@
 import abc
 import time
 from getpass import getpass
-from datetime import datetime
+from datetime import datetime, timezone
+import time
 import imaplib
 import poplib
 import smtplib
@@ -438,7 +439,7 @@ def _create_submission_request(address: str, submission_address: str, fingerprin
     mail['Subject'] = 'WKS submission request'
     mail['To'] = submission_address
     mail['From'] = address
-    mail['Date'] = format_datetime(datetime.utcnow())
+    mail['Date'] = format_datetime(datetime.now(timezone.utc))
     return mail
 
 
@@ -467,7 +468,7 @@ def _create_confirmation_response(address: str, submission: str, nonce: str, fp:
     mail['Subject'] = 'WKS confirmation response'
     mail['To'] = submission
     mail['From'] = address
-    mail['Date'] = format_datetime(datetime.utcnow())
+    mail['Date'] = format_datetime(datetime.now(timezone.utc))
     return mail
 
 
@@ -624,14 +625,14 @@ def main():
         print('Aborted')
         exit(1)
     with incoming_server:
-        now = datetime.utcnow()
+        now = time.monotonic()
         done = False
         request = _create_submission_request(ad, sa, fp, rfprs)
         print('Sending submission request')
         with outgoing_server:
             outgoing_server.send_message(request)
         print('Awaiting response')
-        while not done and (datetime.utcnow() - now).total_seconds() < 300:
+        while not done and time.monotonic() - now < 300:
             time.sleep(5)
             for message in incoming_server.get_new_messages():
                 done = handle_incoming_message(ad, fp, message, outgoing_server)