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

duniterpy/api/bma/blockchain.py

@@ -201,7 +201,7 @@ DIFFICULTIES_SCHEMA = {
         "block": {"type": "number"},
         "levels": {
             "type": "array",
-            "items": [
+            "prefixItems": [
                 {
                     "type": "object",
                     "properties": {