Migrate away from python-gitlab

[tulir]

python-gitlab is not asyncio. Probably easiest to just write the necessary API calls manually into a library file like gitlab_hook.py is for webhooks.

[L0ric0]

woulden't it be better to convert python-gitlab to async? as it is not so big as far a i can see

[tulir]

That would work too, although might be nicer if we can reuse the typed classes in gitlab_hook.py