requirements: Workaround another ImportError when running pytest

When calling ./bin/install from swh-environment in a freshly created venv, for some reason pip will end up installing flask==1.1.2 when processing swh-web (likely related to the venv state after installing other swh modules).

That flask version is quite outdated and will fail to import the json module from the version of itsdangerous installed in the venv.

So as a workaround pin flask to the first version that does not trigger that ImportError issue.

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Migrated from D7207 (view on Phabricator)