swh.web: Adapt to latest storage release_get api change

Related to T645

Test Plan

locally:

• tox (amended with local storage) ok
• pytest

# 420 passed, 7 skipped in 15.20s
_ summary _
...
  flake8: commands succeeded
  mypy: commands succeeded
  py3: commands succeeded

(somehow black always want to reformat everything now).

ci:

• tox (failing until release swh.storage > 0.13.2)

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Migrated from D3854 (view on Phabricator)