Use non-deprecated API for obtaining UTC time

DeprecationWarning: datetime.datetime.utcnow() is deprecated and
scheduled for removal in a future version. Use timezone-aware objects to
represent datetimes in UTC: datetime.datetime.now(datetime.UTC).

Change-Id: Ia2c46fb87c544d98cc2dd68a829f67d4770b479c
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/386615
Tested-by: Łukasz Patron <priv.luk@gmail.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Łukasz Patron <priv.luk@gmail.com>
Commit-Queue: Mike Frysinger <vapier@google.com>
diff --git a/git_config.py b/git_config.py
index 8c2eb6e..c19f766 100644
--- a/git_config.py
+++ b/git_config.py
@@ -795,8 +795,8 @@
                 to be logged.
         """
         self._config = config
-        now = datetime.datetime.utcnow()
-        self._Set("main.synctime", now.isoformat(timespec="microseconds") + "Z")
+        now = datetime.datetime.now(datetime.timezone.utc)
+        self._Set("main.synctime", now.isoformat(timespec="microseconds"))
         self._Set("main.version", "1")
         self._Set("sys.argv", sys.argv)
         for key, value in superproject_logging_data.items():