v0.7.0

Transaction validation 2 0 fix (#12)

* fixed tx validation issues with v2.0 transactions/elections/votes
* added changelog bumped version
* blackified
* using the newest black now
* added method to abstract v2.0, v3.0   differences away
* Added an object version of get_assets
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Signed-off-by: Jürgen Eckel <juergen@riddleandcode.com>