community/py3-gitlab: move from testing

We use this for CI downstream in postmarketOS and it would be useful to have this in community so we in the future can use stable releases for that purpose. Additionally, this package has been in testing for almost two years and has been actively maintained since. As such, I think it makes sense to move this to community.

It was also previously in community, but removed due to a lack of users: f2c7b02b

However, this is clearly not the case any more.

@folliehiyuki does this look okay?