Make pytest plugin optional and provide it in a separate extra requires

This allows tests that require the pytest plugin to depend on swh.journal[pytest] rather than the full swh.journal[testing]. It also avoids importing the whole kafka suite as soon as swh.journal is installed.

assigned to @olasd

requested review from @vlorentz

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• 8c63471d - Make pytest plugin optional and provide it in a separate extra requires

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merged manually