add a script to synchronize labels on all OCA repositories

Here is a script to help definition of labels.

It was already run but on l10n-belgium where I have no access rights.

Here is the talk about labels: https://github.com/OCA/maintainers-tools/issues/29
Used in other places such as https://github.com/OCA/maintainers-tools/blob/master/tools/copy_maintainers.py#L14.
I feel that we can make a constants module and move that there.