bma.blockain.DIFFICULTIES_SCHEMA: Switch to prefixItems (#204)

Which fixes the issue with 2020-12 draft set as default
since jsonschema v4

JSON 2020-12 draft changes tuple from 'items' to 'prefixItems'
https://json-schema.org/draft/2020-12/release-notes.html#changes-to-items-and-additionalitems