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

Close #204 (closed).

changed milestone to %1.2.0

added API: BMA Bug labels

assigned to @moul

changed the description

requested review from @vtexier

approved this merge request

added 2 commits

• 016e7029 - 1 commit from branch main
• 194e9e41 - bma.blockain.DIFFICULTIES_SCHEMA: Switch to prefixItems (#204 (closed))

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enabled an automatic merge when the pipeline for 194e9e41 succeeds

merged