unbreak swh-graph CI

It's broken since 2020-09.

@anlambert tracked the cause to:

14:18 <+anlambert[m]> fyi, swh-graph build failure on Jenkins happens since the upgrade to aiohttp 3.7.0, I managed to bisect the commit that led to the current error: https://github.com/aio-libs/aiohttp/commit/29eccad84e8200b5c90856c8732da0fdbbcef904

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Migrated from T2768 (view on Phabricator)