Use python-gitlab for http requests

Ben Brownrequested to merge
benbrown/python-gitlab into main

Part of #375. Partially helps with #314 (closed), but we can still hit requests timeouts.

This brings in python-gitlab as a dependency and uses it for HTTP requests.

While a bigger refactor would make sense in the future, this at least allows us to quickly make use of python-gitlab's retry logic and error parsing.

Freedesktop SDK's marge-bot deployment has been making use of these changes since 23/08/24.

Edited by Ben Brown

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