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Starting from this version zodbtools requires zodburi to open ZODB
storages by URL (see nexedi/zodbtools@db3b84ec
and nexedi/zodbtools@82b06413)

zodburi in turn requires ZEO (maybe we might patch it to only optionally
depend on it), so we move ZEO requirement from stack/ERP5 to the place
where it is actually first needed.