Public graph/ API does not handle streaming results from endpoints

When playing around with requests.get(stream=True) I noticed that the graph API was hanging and not returning directly a stream as it is supposed to do (eg: on a big query taking ~40s to complete: https://archive.softwareheritage.org/api/1/graph/visit/edges/swh:1:rev:f39d7d78b70e0f39facb1e4fab77ad3df5c52a35?edges=rev:rev).

However, using the API on granet.internal.softwareheritage, the stream is correctly returned, so there is something wrong in between the "two" APIs.

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Migrated from T2900 (view on Phabricator)