use blake2 from py36

[ThomasWaldmann]

we can stop using it from the bundled code or from libb2 library because python3.6+ now has it for us.

[codecov-io]

Codecov Report

Merging #5647 (e9c4085) into master (699256e) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5647      +/-   ##
==========================================
+ Coverage   83.16%   83.29%   +0.12%     
==========================================
  Files          38       38              
  Lines       10139    10139              
  Branches     1680     1680              
==========================================
+ Hits         8432     8445      +13     
+ Misses       1209     1195      -14     
- Partials      498      499       +1     

Impacted Files	Coverage Δ
src/borg/selftest.py	73.80% <100.00%> (ø)
src/borg/archive.py	81.57% <0.00%> (+0.06%)	⬆️
src/borg/helpers/parseformat.py	90.05% <0.00%> (+0.16%)	⬆️
src/borg/repository.py	84.21% <0.00%> (+0.18%)	⬆️
src/borg/archiver.py	80.47% <0.00%> (+0.44%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 699256e...e9c4085. Read the comment docs.