[JIRA] add example jira_add_label_tag.py

[copolycube]

thanks for the discovery of the "black" python formatting tool !

[Spacetown]

@copolycube Joke or serious?

[copolycube]

@Spacetown , no seriously I didn't know that tool.

For the pull request, it's just a basic example I tested since I wasn't sure the Labels (system field) would actually be modifiable the same way a custom field is, thought it could help some other people too, but feel free to reject if it's against your policy.

[gonchik]

Thank you! :)