use blake2 from python and drop our stuff?

[ThomasWaldmann]

master branch has a minimum requirement of python 3.6.

As @enkore already noted in #1044 :

Python 3.6 will include BLAKE2 with a comprehensive API: https://docs.python.org/3.6/library/hashlib.html#blake2

TODO:

• check if we can use it via Python and drop our bundled code / our libb2 requirement / our own wrapper.
• we must have unit tests that make sure both implementations actually yield same hash/mac values.
• use blake2 functions via Python

Bounty: https://www.bountysource.com/issues/96027662-use-blake2-from-python-and-drop-our-stuff

[ThomasWaldmann]

fixed by #5647.