pytest_plugin: Fix cascading assertion error when key is not hashable
I have issues fixing swh-storage!872 (closed) and this revealed itself ;)
Without this commit, we have cascading type error when the assertion is not respected:
*** TypeError: unhashable type: 'dict'
As for example, origin-visit, origin-visit-status,... keys are dict and not bytes.
Test Plan
tox
Migrated from D3249 (view on Phabricator)