ukify: use datetime.timezone.utc instead of datetime.UTC

For compat with older Python versions, since datetime.UTC is an alias
introduced in Python 3.11.

(cherry picked from commit a165918)

src/ukify/ukify.py

@@ -872,7 +872,7 @@ def generate_key_cert_pair(
     # supported/expected:
     # https://learn.microsoft.com/en-us/windows-hardware/manufacture/desktop/windows-secure-boot-key-creation-and-management-guidance?view=windows-11#12-public-key-cryptography
 
-    now = datetime.datetime.now(datetime.UTC)
+    now = datetime.datetime.now(datetime.timezone.utc)
 
     key = rsa.generate_private_key(
         public_exponent=65537,