Argon2 the first part: Implement key derivation (was: part 0)

[hexagonrecursion]

This is the first part of #747. I have added a method to helpers.passphrase.Passphrase to derive keys via argon2.

I have chosen to add a new method instead of changing helpers.passphrase.Passphrase.kdf because:

1. argon2 has more tunable parameters
2. decrypt_key_file needs to be able to select pbkdf2 or argon2 based on key version
3. I have heard that it is usually more pythonic to define two functions than to define one that does two different things based on a flag

PS. After reading the code I have reversed my original decision and started implementing this bottom-up. I'll do decrypt_key_file next
PPS. Sorry for the delay. I am back in your galaxy and ready to work on this today.

[codecov-commenter]

Codecov Report

Merging #6468 (1df8a0c) into master (cc3b5c0) will decrease coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #6468      +/-   ##
==========================================
- Coverage   82.70%   82.68%   -0.02%     
==========================================
  Files          39       39              
  Lines       10519    10525       +6     
  Branches     2076     2076              
==========================================
+ Hits         8700     8703       +3     
- Misses       1311     1314       +3     
  Partials      508      508              

Impacted Files	Coverage Δ
src/borg/helpers/passphrase.py	75.70% <50.00%> (-1.53%)	⬇️

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[ThomasWaldmann]

https://www.ietf.org/archive/id/draft-irtf-cfrg-argon2-03.txt

Select the type y.
If you do not know the difference between them or you consider side-channel attacks
as viable threat, choose Argon2id.

[hexagonrecursion]

ci.appveyor.com:

Uploading artifacts...
Error uploading artifact to the storage: The request was aborted: Could not create SSL/TLS secure channel.

[ThomasWaldmann]

seems like they have issues at appveyor (but we can just ignore that for now).

[ThomasWaldmann]

LGTM!

[ThomasWaldmann]

OK, ready for merging?

[ThomasWaldmann]

I'ld squash the commits together...

[hexagonrecursion]

OK, ready for merging?

ready