Define the pubkey and checksum format in a class

Following #301 (closed).

In the case we want to continue using the <pubkey>:<checksum> checksum system, and not using #341 (closed) base58.b58(de|en)code_check(), defining in DuniterPy the public key and the checksum format in a class would make things clear.

In this class we could clearly define pubkey, checksum variables and methods around them.

Edited Oct 09, 2020 by Moul

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