gitlab.com: Full lister fails to retrieve the range information it needs to start listing

It worked for a time since we did list gitlab instances (T1139). Investigate and fix.

Jun 29 07:34:04 worker01 python3[18335]: [2019-06-29 07:34:04,108: ERROR/ForkPoolWorker-3] Task swh.lister.gitlab.tasks.FullGitLabRelister[25599a68-d227-4d2f-9e94-7345ec7b0d2d] raised unexpected: ValueError('Problem during information fetch: 404',)
                                         Traceback (most recent call last):
                                           File "/usr/lib/python3/dist-packages/celery/app/trace.py", line 382, in trace_task
                                             R = retval = fun(*args, **kwargs)
                                           File "/usr/lib/python3/dist-packages/swh/scheduler/task.py", line 45, in __call__
                                             return super().__call__(*args, **kwargs)
                                           File "/usr/lib/python3/dist-packages/celery/app/trace.py", line 641, in __protected_call__
                                             return self.run(*args, **kwargs)
                                           File "/usr/lib/python3/dist-packages/swh/lister/gitlab/tasks.py", line 47, in full_gitlab_relister
                                             _, total_pages, _ = lister.get_pages_information()
                                           File "/usr/lib/python3/dist-packages/swh/lister/gitlab/lister.py", line 75, in get_pages_information
                                             'Problem during information fetch: %s' % response.status_code)
                                         ValueError: Problem during information fetch: 404

Note:

• This is solely a problem for the main instance gitlab.com. See comments below for more details.

• Other gitlab instances have been unstuck and are currently listed (debian, gnome, gite.lirmm, ...)

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Migrated from T1863 (view on Phabricator)