ci: pr_metadata_check: convert DNM logic to python

GitHub seems to have issue with workflow state caching that causes the
DNM step to not work properly in few cases and not detecting changes in
the DNM tag, forcing people to mess with tags or close/opening PRs,
which in turns restarts all workflows.

Convert the script to Python so that the tag data is guaranteed to be
fresh.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>